Part of farm that's British and a force at sea (4)
I believe the answer is:
barn
'part of farm' is the definition.
'british and a force at sea' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'and' says to put letters next to each other.
'force at sea' becomes 'rn' (abbreviation for Royal Navy).
'b'+'a'+'rn'='BARN'
'that's' is the link.
(Other definitions for barn that I've seen before include "Farm storehouse" , "stable" , "Large outhouse" , "Farm building for storing hay or grain" , "spacious accommodation" .)