Sent in to rearrange sport (6)
I believe the answer is:
tennis
'sport' is the definition.
(tennis is a sport)
'sent in to rearrange' is the wordplay.
'to rearrange' indicates an anagram (I've seen 'rearranged' mean this).
'sent'+'in'='sentin'
'sentin' anagrammed gives 'TENNIS'.
(Other definitions for tennis that I've seen before include "Game, orig. sphairistike" , "Racket sport" , "Evil trap set up for game" , "Sent in (anag.)" , "event" .)