Sing, fluctuating rapidly, unknown line after poem (5)
I believe the answer is:
yodel
'sing fluctuating rapidly' is the definition.
(singing style involving fluctuating between notes)
'unknown line after poem' is the wordplay.
'unknown' becomes 'y' (algebra).
'line' becomes 'l'.
'after' is a charade indicator (letters next to each other).
'poem' becomes 'ode' (ode is a kind of poem).
'l' put after 'ode' is 'odel'.
'y'+'odel'='YODEL'
(Other definitions for yodel that I've seen before include "Sing in high pitch like Swiss mountain dwellers" , "Sing warblingly" , "Sing in a certain way" , "mountain air?" , "Alpine song or call" .)