Knock or stab (4)
I believe the answer is:
pink
I believe this is a double definition.
'knock' is the first definition.
('pink' can be a synonym of 'knock')
'stab' is the second definition.
(I've seen this before)
'or' acts as a link.
(Other definitions for pink that I've seen before include "... similar flower - colour" , "Knock (of engine) - garden flower" , "Left-wing" , "Cut wavy edge on, plant" , "somewhat to the left?" .)