Wrestling in bedroom usually backfiring (4)
I believe the answer is:
sumo
'wrestling' is the definition.
(I've seen this before)
'in bedroom usually backfiring' is the wordplay.
'in' indicates a hidden word.
'backfiring' shows that the letters should be reversed in order.
'omus' is hidden in the letters of 'bedroom usually'.
'omus' back-to-front is 'SUMO'.
(Other definitions for sumo that I've seen before include "Grappling" , "Japanese wrestling for huge men" , "Japanese wrestling with big fat men" , "Jap. wrestling" , "foreign form of fighting" .)