Heard bitter trouble (3)
I believe the answer is:
ail
'trouble' is the definition.
('ail' can be a synonym of 'trouble')
'heard bitter' is the wordplay.
'heard' shows a homophone (sound like).
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'ale' sounds like 'AIL'.
(Other definitions for ail that I've seen before include "Be the worse for wear" , "Afflict; suffer" , "Become unwell" , "Be unwell, suffer" , "Cause to feel indisposed" .)