Weapon found in ditch brought back by petty officer (7)
I believe the answer is:
poniard
'weapon' is the definition.
(poniard is a kind of weapon)
'ditch brought back by petty officer' is the wordplay.
'ditch' becomes 'drain' (I've seen this before).
'brought back' is a reversal indicator.
'by' means one lot of letters go next to another.
'petty officer' becomes 'po' (naval abbreviation).
'drain' reversed gives 'niard'.
'niard' put after 'po' is 'PONIARD'.
'found in' is the link.
(Other definitions for poniard that I've seen before include "Slender dagger" , "A drop in (anag) - dagger" , "Thin knife" , "straight crease?" .)