Lobby in favour of fencing Yorkshire's border (5)
I believe the answer is:
foyer
'lobby' is the definition.
(synonyms)
'in favour of fencing yorkshire's border' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'fencing' is an insertion indicator (to fence can mean to contain or put a limit round).
'border' means to remove the middle letters (I've seen 'borders of' mean this).
'yorkshire' with its centre taken out is 'ye'.
'for' enclosing 'ye' is 'FOYER'.
(Other definitions for foyer that I've seen before include "Entrance hallway" , "Theatre entrance hall" , "Anteroom" , "Reception room or waiting area" , "Reception lobby" .)