Heard beer was trouble (3)
I believe the answer is:
ail
'trouble' is the definition.
(I know that trouble can be written as ail)
'heard beer' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'beer' becomes 'ale' (type of beer).
'ale' is a homophone of 'AIL'.
'was' acts as a link.
(Other definitions for ail that I've seen before include "Become sick" , "Trouble, afflict; be sickly" , "Cause to feel indisposed" , "upset" , "pain" .)