Song West Side Story heroine left unstarted (4)
I believe the answer is:
aria
'song' is the definition.
(piece of music in an opera)
'west side story heroine left unstarted' is the wordplay.
I cannot really understand how this works, but
'side' could be 'r' (abbreviation for right) and 'r' is found within the answer.
The remaining letters 'aia' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aria that I've seen before include "Repeated strain" , "Operatic air" , "Operatic number" , "Operatic vocal" , "Song such as "Your Tiny Hand is Frozen"" .)