Numbers sent off (4)
I believe the answer is:
tens
'numbers' is the definition.
(ten is a kind of number)
'sent off' is the wordplay.
'off' indicates anagramming the letters.
'sent' anagrammed gives 'TENS'.
(Other definitions for tens that I've seen before include "Half-scores" , "X, X, X, ..." , "At least 20" , "Cardinal numbers" .)