He has to live at the Nag's Head (3)
I believe the answer is:
ben
'he' is the definition.
(I've seen this in another clue)
'live at the nag's head' is the wordplay.
'live' becomes 'be' (synonyms).
'at' means one lot of letters go next to another.
'head' indicates taking the first letters.
The initial letter of 'nags' is 'n'.
'be'+'n'='BEN'
'has to' 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" .)