Understand an old rocker with very little energy at first did nothing (9)
'did nothing' is the definition.
'understand an old rocker with very little energy at first' is the wordplay.
'understand' becomes 'get' (getting is a kind of understanding).
'an' becomes 'a'.
'old rocker' becomes 'ted'.
'with very little energy' becomes 've' (I can't explain this - if you can you should believe this answer much more).
'at first' means one lot of letters go next to another.
'getated' after 've' is 'VEGETATED'.