For a considerable period, it's fun (4,4)
I believe the answer is:
good time
'fun' is the definition.
(thesaurus)
'for a considerable period' is the wordplay.
I cannot really understand how this works, but
'considerable' could be 'good' (I've seen this in another clue) and 'good' is located in the answer.
'period' could be 'time' (period of time) and 'time' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'it's' is the link.
Can you help me to learn more?
(Other definitions for good time that I've seen before include "Seeking pleasure" , "in which one is early" .)