Say, grow old and mean (7)
I believe the answer is:
average
'mean' is the definition.
(synonyms)
'say grow old' is the wordplay.
'say' becomes 'aver' (synonyms).
'grow old' becomes 'age' (synonyms).
'aver'+'age'='AVERAGE'
'and' acts as a link.
(Other definitions for average that I've seen before include "'Medium, middling (7)'" , "just like any other" , "Arithmetical mean" , "Mean, so-so" , "Fair to middling" .)