Pressure injected in to overhaul weapon (7)
I believe the answer is:
torpedo
'weapon' is the definition.
(weapon used at sea)
'pressure injected in to overhaul' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'injected in' means one lot of letters goes inside another.
'overhaul' becomes 're-do' (I've seen this in another clue).
'to'+'redo'='toredo'
'p' put into 'toredo' is 'TORPEDO'.
(Other definitions for torpedo that I've seen before include "American sandwich made with a long split roll" , "To wreck" , "Sea-borne missile" , "Trooped (anag.)" , "electric ray" .)