Sing unknown poem by lake (5)
I believe the answer is:
yodel
'sing' is the definition.
(I know that yodel is a type of sing)
'unknown poem by lake' is the wordplay.
'unknown' becomes 'y' (algebra).
'poem' becomes 'ode' (ode is a kind of poem).
'by' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'y'+'ode'+'l'='YODEL'
(Other definitions for yodel that I've seen before include "Swiss warble" , "Alpine warble" , "Sing in a certain way" , "Sing falsetto like Swiss mountain people" , "mountain air?" .)