Sing unknown poem by student (5)
I believe the answer is:
yodel
'sing' is the definition.
(yodeling is a kind of singing)
'unknown poem by student' 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.
'student' becomes 'L' (as in L-plates for learner drivers).
'y'+'ode'+'l'='YODEL'
(Other definitions for yodel that I've seen before include "Sing in traditional Swiss manner" , "Sing in high trilling tone like Swiss mountaineer" , "Vocalise" , "Alpine mountain call" , "Sing partly falsetto" .)