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 can't explain this - if you can you should believe this answer much more).
'a'+'g'+'es'='AGES'
'for' acts as a link.
(Other definitions for ages that I've seen before include "Historic periods or prolonged time" , "Awfully long time" , "Matures" , "Aeons" , "Donkey's years" .)