He has to live at the Nag's Head (3)
I believe the answer is:
ben
'he' is the definition.
'to live at the nag's head' is the wordplay.
'to live' becomes 'be' ('be' can be a synonym of 'live').
'at' says to put letters next to each other.
'head' says to take the initial letters.
The first letter of 'nags' is 'n'.
'be'+'n'='BEN'
'has' acts as a link.
(Other definitions for ben that I've seen before include "A Big London clock tower" , "Mountain peak (Scot.)" , "Battle" , "The boy from the Scottish mountains" , "inner room in Oban, say" .)