For a model, is once enough? (7)
I believe the answer is:
example
'for a model' is the definition.
(synonyms)
'once enough?' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'enough?' becomes 'ample' (ample means enough or plenty).
'ex'+'ample'='EXAMPLE'
'is' acts as a link.
(Other definitions for example that I've seen before include "Thing characteristic of its kind" , "Illustration or paradigm" , "Illustrative case" , "Instance or illustration" , "warning" .)