Sound, perhaps, if no longer fancy? (2,3)
I believe the answer is:
go off
'fancy?' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'sound perhaps if no longer' is the wordplay.
I cannot really see how this works, but
'sound' could be 'go' (synonyms) and 'go' is present in the answer.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for go off that I've seen before include "Explode" , "Fire" , "discharged" , "fall asleep" , "Turn" .)