Piano man's mate (3)
I believe the answer is:
pal
'mate' is the definition.
(both can mean a friend)
'piano man's' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'man' becomes 'al' (man's name).
'p'+'al'='PAL'
(Other definitions for pal that I've seen before include "Crony" , "Mate, informal friend" , "Buddy, chum" , "Mate, buddy" , "Brother" .)