Musical note for one singer mostly (3)
I believe the answer is:
sol
'musical note' is the definition.
(I've seen this before)
'for one singer mostly' is the wordplay.
'for one singer' becomes 'solo' (as in a solo performance).
'mostly' means to remove the last letter.
'solo' with its final letter removed is 'SOL'.
(Other definitions for sol that I've seen before include "Personification of the sun" , "nearby star" , "A Latin sun" , "Roman counterpart of Helios" , "Musical note" .)