Reform aid lends Labour's leader overwhelming majority (9)
I believe the answer is:
landslide
'majority' is the definition.
(I have seen 'big majority' mean 'landslide' so perhaps 'majority' could also mean 'landslide')
'reform aid lends labour's leader overwhelming' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'labour's leader' becomes 'l'.
'overwhelming' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'aid'+'lends'='aidlends'
'aidlends' with letters rearranged gives 'andslide'.
'andslide' after 'l' is 'LANDSLIDE'.
(Other definitions for landslide that I've seen before include "convincing victory" , "great electoral success" , "Large earth movement" , "significant win in contest" , "big victory" .)