Low communist tied up (6)
I believe the answer is:
moored
'tied up' is the definition.
(I've seen this in another clue)
'low communist' is the wordplay.
'low' becomes 'moo' (both can mean to make a cow noise).
'communist' becomes 'red' (red can informally mean communist).
'moo'+'red'='MOORED'
(Other definitions for moored that I've seen before include "Made fast" , "Of boat, tied up or secured" , "Fastened with a line or anchored" , "Of boat, secured by rope to shore" , "Secured by rope or cable" .)