Popular Northern pub (3)
I believe the answer is:
inn
'pub' is the definition.
(an inn is a type of pub)
'popular northern' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'in'+'n'='INN'
(Other definitions for inn that I've seen before include "Pub with lodging" , "Gray's, Lincoln's" , "Possibly Jamaica" , "Drinking-place" , "eg the Tabard" .)