Chap with mum at the Nag's Head (3)
I believe the answer is:
man
'chap' is the definition.
(chap can mean a man)
'mum at the nag's head' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'at' means one lot of letters go next to another.
'head' says to take the initial letters.
The first letter of 'nags' is 'n'.
'ma'+'n'='MAN'
'with' acts as a link.
(Other definitions for man that I've seen before include "Where people compete to win TT" , "Adult human male" , "Attendant" , "Vassal; chess piece" , "Isle of -; chess piece" .)