Tied up by a low leftist! (6)
I believe the answer is:
moored
'tied up' is the definition.
(I've seen this in another clue)
'by a low leftist' is the wordplay.
'by' means one lot of letters go next to another.
'a low' becomes 'moo' (both can mean to make a cow noise).
'leftist' becomes 'red' (red can informally mean communist).
'moo' put next to 'red' is 'MOORED'.
(Other definitions for moored that I've seen before include "Tied up to berth like ship or boat" , "Parked (of a boat)" , "Made fast" , "Secured by rope or cable" , "Of boat, secured by rope to shore" .)