Gets on for a good while (4)
I believe the answer is:
ages
'gets on' is the definition.
(get on can mean to get older)
'a good while' is the wordplay.
'good' becomes 'g' (abbreviation).
'while' becomes 'es' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'g'+'es'='AGES'
'for' is the link.
(Other definitions for ages that I've seen before include "Historic periods or prolonged time" , "Awfully long time" , "Matures" , "Aeons" , "Donkey's years" .)