Mars alien appears after communist plot (3,6)
I believe the answer is:
red planet
'mars' is the definition.
(synonyms)
'alien appears after communist plot' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'appears after' says to put letters next to each other.
'communist' becomes 'red' (red can informally mean communist).
'plot' becomes 'plan' (plot can mean to conspire or plan).
'red'+'plan'='redplan'
'et' put after 'redplan' is 'RED PLANET'.
(Other definitions for red planet that I've seen before include "Informal name for Mars" , "one of our neighbours" , "Mars is colourfully known as the . . . . . . . . ." , "object in 21 [SPACE]" , "Colourful description of Mars" .)