Illustration no longer sufficient? (7)
I believe the answer is:
example
'illustration' is the definition.
(synonyms)
'no longer sufficient?' is the wordplay.
'no longer' becomes 'ex'.
'sufficient?' becomes 'ample' (both can mean enough).
'ex'+'ample'='EXAMPLE'
(Other definitions for example that I've seen before include "Illustrative exercise" , "Someone to emulate" , "Standard" , "Something illustrating a general rule" , "Thing characteristic of its kind" .)