Illustration formerly sufficient (7)
I believe the answer is:
example
'illustration' is the definition.
(I know that illustration can be written as example)
'formerly sufficient' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'sufficient' becomes 'ample' (both can mean enough).
'ex'+'ample'='EXAMPLE'
(Other definitions for example that I've seen before include "Something to be imitated" , "Characteristic item" , "Illustrative exercise" , "Typical specimen" , "Instance - pattern" .)