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 it's' is the wordplay.
I cannot really understand how this works, but
'period' could be 'go' (go is a kind of period) and 'go' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This explanation may well be incorrect...
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" .)