Moaning endlessly in Cabinet about the Opposition? (12)
I believe the answer is:
inconsolable
'moaning endlessly' is the definition.
(unable to be consoled)
'in cabinet about the opposition?' is the wordplay.
'cabinet' becomes 'console' (a console is a cabinet for storing TV equipment).
'about' is an insertion indicator.
'the opposition?' becomes 'Lab' (Labour Party, UK opposition party).
'console' going around 'lab' is 'consolable'.
'in'+'consolable'='INCONSOLABLE'
(Other definitions for inconsolable that I've seen before include "Not open to being comforted" , "not easily made cheerful" , "Not able to be comforted" , "Unable to be comforted" , "Heartbroken" .)