Crack marksman departed with detective (7)
I believe the answer is:
deadeye
'crack marksman' is the definition.
(informal term for an expert marksman)
'departed with detective' is the wordplay.
'departed' becomes 'dead' (similar in meaning).
'with' says to put letters next to each other.
'detective' becomes 'eye' (I've seen this in other clues).
'dead'+'eye'='DEADEYE'
(Other definitions for deadeye that I've seen before include "Dick" , "One secures a shroud" , "Wooden block to take a lanyard" , "Unerring marksman" , "top marksman" .)