Lament what willows do? (4)
I believe the answer is:
weep
This clue is a double definition.
'lament' is the first definition.
(thesaurus)
'what willows do?' is the second definition.
(weeping willow is a type of willow tree)
(Other definitions for weep that I've seen before include "Turn on the waterworks" , "leak" , "pipe" , "Bawl" , "Cry, shed tears" .)