Serving American in pub is reasonable (7)
I believe the answer is:
logical
'reasonable' is the definition.
('logical' can be similar in meaning to 'reasonable')
'serving american in pub' is the wordplay.
'serving american' becomes 'gi' (I've seen this in other clues).
'in' is an insertion indicator.
'pub' becomes 'local' (I've seen this before).
'gi' inserted within 'local' is 'LOGICAL'.
'is' acts as a link.
(Other definitions for logical that I've seen before include "Using reason" , "According with reason" , "Following correct reasoning" , "Well reasoned" , "Correctly reasoned, makes sense" .)