Piano: one for each musician (5)
I believe the answer is:
piper
'musician' is the definition.
(piper is a kind of musician)
'piano one for each' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'one' becomes 'i' (Roman numeral).
'for each' becomes 'per' (I've seen this in another clue).
'p'+'i'+'per'='PIPER'
(Other definitions for piper that I've seen before include "eg Flautist" , "who puffs out his cheeks?" , "One plays" , "One playing wind instrument" , "Musician sometimes paid (or pied!)" .)