Jimmy is definitely part of the song, but I think it's more about the start of her solo career. I wrote this interpretation (opinion, folks) of "Straight Back" a while back. If anyone has an interview of Stevie talking about "Straight Back," please share.
About the Song
Though Stevie has never revealed the meaning of the song, “Straight Back” is interpreted to be about the start of her solo career, characterized as “the dream.”
The opening lyrics suggest her complicated relationship with Fleetwood Mac, compared here to the strategy of a card game, like poker.
What can I say this time
Which card shall I play
“The dream is not over” seems to refer to Stevie feeling hopeful about her solo career after the success of Bella Donna; and “the dream is just away” and “you will fly like some little wing straight back to the sun” revealing Stevie’s intention to return to “the dream,” or her solo career, at some point.
The second verse may allude to Stevie secretly writing songs on the piano for her second solo album, The Wild Heart (1983), during the 1982 Mirage recording sessions. “Like a wolf on the run,” Stevie had to be discreet and strategic about her solo career plans, so not to jeopardize her future with Fleetwood Mac.
Fingers find the ivory keys
And a song begins to begin
Like a wolf on the run
“The dream” also seems to represent independence, perhaps artistic freedom from Fleetwood Mac. Having written so many songs only to be unused or discarded during her formative years in Fleetwood Mac, Stevie may have temporarily lost sight of her goal to be an independent, prolific songwriter.
And you will find while in the wind
Something that you lost
The dream was never over, no
The dream was only lost
Who Stevie is referring to in the bridge of the song is unclear. The implication seems to be Fleetwood Mac, and “how good it can be” when they are creating music together.
She remembers how good it can be
He remembers a melody
He remembers how good it can be
She remembers a melody
This lyric is reminiscent of “come in out of the darkness” from “Bella Donna.” It could represent the spirit of shining through the darkness, gloom, or despair.
In the shadow of my shadow in a gleam
The following lyrics could be interpreted as Fleetwood Mac waiting “hours and hours” for Stevie to return to the band so they could start recording Mirage, despite “the dream” having just begun. “So strong and fleeting” might represent how quickly “the dream” — the tremendous success of Bella Donna — came and went.
Hours and hours of waiting for you
So strong and so fleeting
The dream has just begun
Hours and hours of waiting this way
Meeting me
__________________
Daniel 
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