Sing unknown poem by learner (5)
I believe the answer is:
yodel
'sing' is the definition.
(I know that yodel is a kind of sing)
'unknown poem by learner' is the wordplay.
'unknown' becomes 'y' (algebra).
'poem' becomes 'ode' (ode is a type of poem).
'by' means one lot of letters go next to another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'y'+'ode'+'l'='YODEL'
(Other definitions for yodel that I've seen before include "Sing (Swiss style)" , "Swiss warble" , "Sing (Alpine folk music)" , "Sing like the Lonely Goatherd?" , "Falsetto singing" .)