Old enough, for instance (7)
I believe the answer is:
example
'instance' is the definition.
(synonyms)
'old enough' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'enough' becomes 'ample' (ample means enough or plenty).
'ex'+'ample'='EXAMPLE'
'for' is the link.
(Other definitions for example that I've seen before include "Model - instance" , "lesson" , "Prototype" , "Specimen, illustration" , "Specimen, sample" .)