Detectives heading off? Agreed (3)
I believe the answer is:
yes
'agreed' is the definition.
('agreed' can mean 'yes')
'detectives heading off?' is the wordplay.
'detectives' becomes 'eyes' (I have seen 'private detectives' mean 'eyes' so perhaps 'detectives' could also mean 'eyes').
'heading off?' means to remove the first letter.
'eyes' with its initial letter taken away is 'YES'.
(Other definitions for yes that I've seen before include "That's 27 [RIGHT]" , "Affirmative answer" , "Validating vote" , "Oui" , "Affirmative response" .)