In place of carnival, time for something very amusing (4)
I believe the answer is:
riot
'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'place of carnival time for something very amusing' is the wordplay.
I cannot quite see how this works, but
'place' could be 'rio' (I have seen 'place of carnival' mean 'rio' so perhaps 'place' could also mean 'rio') and 'rio' is present in the answer.
'time' could be 't' (abbreviation) and 't' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for riot that I've seen before include "Ruckus, insurrection" , "loose living" , "Wild revelry" , "Uproar" , "Disturbance by unruly mob" .)