Go off and film rat (8)
I believe the answer is:
turncoat
'rat' is the definition.
(I've seen this before)
'go off and film' is the wordplay.
'go off' becomes 'turn' (I've seen this before).
'and' means one lot of letters go next to another.
'film' becomes 'coat' (I've seen this in other clues).
'turn'+'coat'='TURNCOAT'
(Other definitions for turncoat that I've seen before include "'Renegade, traitor (8)'" , "One betraying party" , "Rat" , "Deserter to opposition" , "Judas" .)