Undercoat for Communist captain (3,4)
I believe the answer is:
red lead
'captain' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'undercoat for communist' is the wordplay.
'undercoat' becomes 'lead' (I am not sure about this - if you are sure you should believe this answer much more).
'for' says to put letters next to each other (I've seen this in other clues).
'communist' becomes 'red' (red can informally mean communist).
'lead' after 'red' is 'RED LEAD'.
Can you help me to learn more?
(Other definitions for red lead that I've seen before include "Metal oxide used as pigment" , "Paint pigment" .)