Suffering under Labour leader's decline (8)
I believe the answer is:
languish
'decline' is the definition.
(I've seen this before)
'suffering under labour leader's' is the wordplay.
'suffering' becomes 'anguish' (anguish is a kind of suffering**).
'under' is a charade indicator (letters next to each other).
'leader' says to take the initial letters.
The initial letter of 'labour' is 'l'.
'anguish' after 'l' is 'LANGUISH'.
(Other definitions for languish that I've seen before include "Become discouraged" , "Grow weak or feeble" , "Pine, waste away" , "go into decline" , "Suffer hardship" .)