Unknown poem left to sing (5)
I believe the answer is:
yodel
'sing' is the definition.
(I know that yodel is a more specific form of the action sing)
'unknown poem left' is the wordplay.
'unknown' becomes 'y' (algebra).
'poem' becomes 'ode' (ode is a type of poem).
'left' becomes 'l' (common abbreviation).
'y'+'ode'+'l'='YODEL'
'to' is the link.
(Other definitions for yodel that I've seen before include "Sing partly falsetto" , "Falsetto singing" , "Sing (Swiss style)" , "Practise Alpine singing" , "Vocalise" .)