Cage fighting? RIP then junior (6)
I believe the answer is:
prison
'cage' is the definition.
(I've seen this before)
'fighting? rip then junior' is the wordplay.
'fighting?' indicates anagramming the letters.
'then' is a charade indicator (letters next to each other).
'junior' becomes 'son' (I've seen this in another clue).
'rip' anagrammed gives 'pri'.
'pri'+'son'='PRISON'
(Other definitions for prison that I've seen before include "Correctional institution" , "Slammer" , "Eg, Wormwood Scrubs" , "Penitentiary" , "Place of confinement" .)