A soldier, British, returning to secure space on ship for gun (3,6)
I believe the answer is:
big bertha
'gun' is the definition.
(thesaurus)
'a soldier british returning to secure space on ship' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'returning' is a reversal indicator.
'to secure' is an insertion indicator (inserted letters are secured inside).
'space on ship' becomes 'berth' (a berth is a bunk on a ship).
'a'+'gi'+'b'='agib'
'agib' reversed gives 'biga'.
'biga' going around 'berth' is 'BIG BERTHA'.
'for' is the link.
(Other definitions for big bertha that I've seen before include "WWI German siege gun" , "cannon" , "Great War German gun" .)