Greek character joins American returning on musical instrument (5)
I believe the answer is:
piano
'musical instrument' is the definition.
'piano' can be an answer for 'instrument' (piano is a kind of instrument). I also know that 'musical' relates to this answer.
'greek character joins american returning on' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'joins' says to put letters next to each other.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'returning' shows that the letters should be reversed in order.
'on' written backwards gives 'no'.
'pi'+'a'+'no'='PIANO'
(Other definitions for piano that I've seen before include "Softly (mus.)" , "key holder" , "Stringed instrument - to be played quietly" , "Softly, musically speaking" , "Quiet (music)" .)