Left-wing member associated with Keir originally (4)
I believe the answer is:
pink
'left-wing' is the definition.
(I've seen this before)
'member associated with keir originally' is the wordplay.
'member' becomes 'pin' (pin is a kind of member).
'associated with' is a charade indicator (letters next to each other).
'originally' suggests taking the first letters.
The initial letter of 'keir' is 'k'.
'pin'+'k'='PINK'
(Other definitions for pink that I've seen before include "Comedy films involving a diamond, Inspector Clouseau and Peter Sellers" , "... similar flower - colour" , "Knock (of engine) - garden flower" , "(Shade of) flower" , "Left-wing" .)