Soldiers in depot assembled missile (7)
I believe the answer is:
torpedo
'missile' is the definition.
(I've seen this before)
'soldiers in depot assembled' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'in' is an insertion indicator.
'assembled' indicates an anagram.
'depot' anagrammed gives 'tpedo'.
'or' placed inside 'tpedo' is 'TORPEDO'.
(Other definitions for torpedo that I've seen before include "Missile form a submarine" , "Naval weapon" , "ruin" , "American sandwich made with a long split roll" , "Trooped (anag.)" .)