Vocalise Southey’s last poem on lake (5)
I believe the answer is:
yodel
'vocalise' is the definition.
(I've seen this before)
'southey's last poem on lake' is the wordplay.
'last' says to take the final letters.
'poem' becomes 'ode' (ode is a kind of poem).
'on' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
The last letter of 'southey' is 'y'.
'y'+'ode'+'l'='YODEL'
(Other definitions for yodel that I've seen before include "Sing like Frank Ifield?" , "Sing in a certain way" , "unusual singing" , "Sing partly falsetto" , "Sing (Swiss style)" .)