Recce ship’s officer’s gun enclosure (8)
I believe the answer is:
casemate
'gun enclosure' is the definition.
(enclosure from which guns are fired)
'recce ship's officer's' is the wordplay.
'recce' becomes 'case' (both can mean to investigate a location).
'ship's officer' becomes 'mate' (mate is a kind of ship's officer).
'case'+'mate'='CASEMATE'
(Other definitions for casemate that I've seen before include "part of fortress" , "Chamber" , "fortified part of building" .)