Yeats's top poem with Latin form of singing (5)
I believe the answer is:
yodel
'form of singing' is the definition.
'yeats's top poem with latin' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'poem' becomes 'ode' (ode is a type of poem).
'with' means one lot of letters go next to another.
'latin' becomes 'l'.
The first letter of 'yeats' is 'y'.
'y'+'ode'+'l'='YODEL'
(Other definitions for yodel that I've seen before include "Sing using falsetto" , "Sing like Frank Ifield?" , "Alpine warble" , "Sing in Tyrolean fashion" , "Sing like the Lonely Goatherd?" .)