Unknown poem has line to sing (5)
I believe the answer is:
yodel
'sing' is the definition.
(yodeling is a kind of singing)
'unknown poem has line' is the wordplay.
'unknown' becomes 'y' (algebra).
'poem' becomes 'ode' (ode is a kind of poem**).
'has' says to put letters next to each other.
'line' becomes 'l'.
'y'+'ode'+'l'='YODEL'
'to' acts as a link.
(Other definitions for yodel that I've seen before include "Sing by alternating between normal voice and falsetto" , "Sing like some Alpine dwellers" , "Loftily call" , "Falsetto singing" , "Sing in Tyrolean fashion" .)