Not out having fun? It's early enough (2,4,4)
I believe the answer is:
in good time
'early enough' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?
'not out having fun?' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'having fun?' becomes 'goodtime' (I am not sure about this - if you are sure you should believe this answer much more).
'in'+'goodtime'='IN GOOD TIME'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for in good time that I've seen before include "How the drummer should play" , "Quite early enough" , "Certainly not late" , "Quickly" , "Without risk of being late" .)