Blush on seeing Communist study (6)
I believe the answer is:
redden
'blush' is the definition.
(I know that blush is a more specific form of the action redden)
'on seeing communist study' is the wordplay.
'on' says to put letters next to each other.
'seeing communist' becomes 'red' (red can informally mean communist).
'study' becomes 'den' (term for a study or office).
'red' next to 'den' is 'REDDEN'.
(Other definitions for redden that I've seen before include "Give visible sign of embarrassment" , "Blush like the dawn sky" , "Reveal embarrassment in colourful way" , "Blush or become flushed" , "Blush, colour accordingly" .)