Nick is residing in flat, briefly (6)
I believe the answer is:
prison
'nick' is the definition.
(British slang)
'is residing in flat briefly' is the wordplay.
I cannot really understand how this works, but
'is' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for prison that I've seen before include "Penitentiary" , "Cooler" , "Place of incarceration" , "Place of detention for criminals" , "No rips (anag)" .)