Private Investigators told to drop last gun (6)
I believe the answer is:
pistol
'gun' is the definition.
(pistol is a kind of gun)
'private investigators told to drop last' is the wordplay.
I cannot really see how this works, but
'to' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pistol that I've seen before include "Officer's side-arm" , "Small gun" , "Piece" , "A small hand-gun" , "Hand weapon" .)