Dead old detective backed missile (6)
I believe the answer is:
exocet
'missile' is the definition.
(I've seen this before)
'dead old detective backed' is the wordplay.
'dead' becomes 'ex'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detective' becomes 'tec' ('tec' can be a synonym of 'detective'**).
'backed' says the letters should be written in reverse.
'tec' back-to-front is 'cet'.
'ex'+'o'+'cet'='EXOCET'
(Other definitions for exocet that I've seen before include "Type of guided missile" , "Short-range guided missile" , "French anti-ship guided missile" , "Tactical missile" , "French missile used by Argentina in the Falklands War" .)