At the Nag's Head, he's the master (3)
I believe the answer is:
man
'the master' is the definition.
(thesaurus)
'at the nag's head he's' is the wordplay.
'at' means one lot of letters go next to another.
'head' suggests taking the first letters.
'he' becomes 'ma' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'nags' is 'n'.
'n' put after 'ma' is 'MAN'.
Can you help me to learn more?
(Other definitions for man that I've seen before include "just one" , "Fellow" , "Male person" , "Staff the island" , "race" .)