A bullet for the detective (6)
I believe the answer is:
tracer
I believe this clue is a double definition.
'a bullet' is the first definition.
(I've seen this before)
'the detective' is the second definition.
(I've seen this in another clue)
'for' is the link.
(Other definitions for tracer that I've seen before include "ammunition" , "Type of bullet or shell" , "Bullet whose track can be seen - element tracking radiation" , "Kind of bullet that leaves trail" , "He's managed to find" .)