Lament onset of war and trouble (4)
I believe the answer is:
wail
'lament' is the definition.
('wail' can be a synonym of 'lament')
'onset of war and trouble' is the wordplay.
'onset of' suggests taking the first letters.
'and' is a charade indicator (letters next to each other).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
The initial letter of 'war' is 'w'.
'w'+'ail'='WAIL'
(Other definitions for wail that I've seen before include "Lamentation" , "Howl" , "Lament loudly" , "Cry of lament" , "Utter a long sad cry" .)