Key to enter communist plant (4)
I believe the answer is:
reed
'plant' is the definition.
(I know that reed is a type of plant)
'key to enter communist' is the wordplay.
'key' becomes 'e' (musical key).
'to enter' indicates putting letters inside.
'communist' becomes 'red' (red can informally mean communist).
'e' put into 'red' is 'REED'.
(Other definitions for reed that I've seen before include "Marsh plant with straight hollow stem" , "Blade of tall marsh grass" , "A blade of march grass" , "Oliver, say" , "Rush" .)