Native British men in pub (6)
I believe the answer is:
inborn
'native' is the definition.
(synonyms)
'british men in pub' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'men' becomes 'OR' (Other Ranks: non-commissioned soldiers).
'in' means one lot of letters goes inside another.
'pub' becomes 'inn' (an inn is a type of pub).
'b'+'or'='bor'
'bor' inserted into 'inn' is 'INBORN'.
(Other definitions for inborn that I've seen before include "naturally!" , "Native" , "Congenital" , "that's natural" , "Normally existing at birth" .)