After refusal, detective signals assent (4)
I believe the answer is:
'signals assent' is the definition.
(I've seen this before)
'after refusal detective' is the wordplay.
'after' says to put letters next to each other.
'refusal' becomes 'no'.
'detective' becomes 'ds' (detective sergeant).
'no' next to 'ds' is 'NODS'.
(Other definitions for nods that I've seen before include "Agrees" , "is about to crash?" , "Shows assent" , "Affirmative gestures" , "Is careless" .)