He has a girl at Maidenhead (4)
I believe the answer is:
adam
'he' is the definition.
'a girl at maidenhead' is the wordplay.
'a girl' becomes 'ada' (girl's name).
'at' is a charade indicator (letters next to each other).
'maidenhead' becomes 'm' (head letter of 'maiden').
'ada'+'m'='ADAM'
'has' is the link.
(Other definitions for adam that I've seen before include "First guy" , "First man in the Bible" , "Father of Cain and Abel" , "One of our common ancestors?" , "Robert ..., ... Clayton" .)