An illustration no longer large enough (7)
I believe the answer is:
example
'an illustration' is the definition.
('example' can be a synonym of 'illustration')
'no longer large enough' is the wordplay.
'no longer' becomes 'ex'.
'large enough' becomes 'ample' (I've seen this before).
'ex'+'ample'='EXAMPLE'
(Other definitions for example that I've seen before include "Paradigm or model" , "Instance or illustration" , "Case in point" , "lesson" , "Illustration or model" .)