One's to get in beer? Gangway! (5)
I believe the answer is:
aisle
'gangway' is the definition.
('aisle' can be a synonym of 'gangway')
'one's to get in beer?' is the wordplay.
'one's' becomes 'is' (roman numeral).
'to get in' indicates putting letters inside.
'beer?' becomes 'ale' (I've seen this before).
'is' placed into 'ale' is 'AISLE'.
(Other definitions for aisle that I've seen before include "Passage between blocks of chairs" , "Passage between seating areas" , "Passage between shelves in supermarket" , "Path between seats in church" , "Divide in a church" .)