Sign rejected in Benedictine monasteries (4)
I believe the answer is:
omen
'sign' is the definition.
(I've seen this before)
'rejected in benedictine monasteries' is the wordplay.
'rejected' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'nemo' can be found hidden inside 'benedictine monasteries'.
'nemo' backwards is 'OMEN'.
(Other definitions for omen that I've seen before include "Sign of a future event, good or bad" , "Portent of evil" , "Forewarning, harbinger" , "Prognostication" , "premonition" .)