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