Communist behemoths are stars (3,6)
I believe the answer is:
red giants
'are stars' is the definition.
(type of star)
'communist behemoths' is the wordplay.
'communist' becomes 'red' (red can informally mean communist).
'behemoths' becomes 'giants' ('giant' can be a synonym of 'behemoth').
'red'+'giants'='RED GIANTS'
(Other definitions for red giants that I've seen before include "Very large stars" , "Stars of high luminosity" .)