Weapon detective backed a great deal (6)
I believe the answer is:
pistol
'weapon' is the definition.
(pistol is a kind of weapon)
'detective backed a great deal' is the wordplay.
'detective' becomes 'pi' (I've seen this in other clues).
'backed' says the letters should be written in reverse.
'a great deal' becomes 'lots' (I've seen this before).
'lots' reversed gives 'stol'.
'pi'+'stol'='PISTOL'
(Other definitions for pistol that I've seen before include "A small hand-gun" , "Type of gun" , "Arm" , "Hand weapon" , "Shakespearean - weapon" .)