The most common and most likely answer to this crossword clue is the 6 letter word SWAYED
I've seen this crossword clue in The Guardian
S
W
A
Y
E
D
Buddy explains!

The straight part
We know that "Was a rocker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was a rocker = swayed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SWAYED mean?
SWAYED verb- To move or swing from side to side; or backward and forward; to rock.
- To move or wield with the hand; to swing; to wield.
- To influence or direct by power, authority, persuasion, or by moral force; to rule; to govern; to guide. Compare persuade.
- To cause to incline or swing to one side, or backward and forward; to bias; to turn; to bend; warp.
- To hoist (a mast or yard) into position.
- To be drawn to one side by weight or influence; to lean; to incline.
- To have weight or influence.
- To bear sway; to rule; to govern.
- Bent backwards, as in swayback