Sent off traps (4)
I believe the answer is:
nets
'traps' is the definition.
(I know this)
'sent off' is the wordplay.
'off' indicates an anagram.
'sent' anagrammed gives 'NETS'.
(Other definitions for nets that I've seen before include "'Traps for 29 dn, say (4)'" , "bags" , "Catches (fish)" , "Practice pitches (cricket)" , "Traps, to catch fish say" .)