Tied up a low leftist (6)
I believe the answer is:
moored
'tied up' is the definition.
'moored' can be an answer for 'tied' (thesaurus). I'm not sure about the 'up' bit.
'a low leftist' is the wordplay.
'a low' becomes 'moo' (both can mean to make the noise of a cow).
'leftist' becomes 'red' (red is a kind of leftist).
'moo'+'red'='MOORED'
Can you help me to learn more?
(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" .)