Nickname girl heard (8)
I believe the answer is:
monicker
'nickname' is the definition.
(a monicker is a name or nickname)
'girl heard' is the wordplay.
'girl' becomes 'Monica' (girl's name).
'heard' indicates a 'sounds like' (homophone) clue.
'monica' sounds like 'MONICKER'.
(Other definitions for monicker that I've seen before include "Handle" , "Nickname (slang)" , "Name (sl)" , "Name or nickname" .)