Fillet pun failed, and that's being generous (9)
I believe the answer is:
plentiful
'that's being generous' is the definition.
(thesaurus)
'fillet pun failed' is the wordplay.
'failed' is an anagram indicator.
'fillet'+'pun'='filletpun'
'filletpun' with letters rearranged gives 'PLENTIFUL'.
'and' acts as a link.
(Other definitions for plentiful that I've seen before include "Existing in great quantities" , "common" , "Productive" , "Abundant - profuse" , "Abundant, copious" .)