Low loop for tying up (7)
I believe the answer is:
mooring
'tying up' is the definition.
(synonyms)
'low loop' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'loop' becomes 'ring' (I've seen this in other clues).
'moo'+'ring'='MOORING'
'for' acts as a link.
(Other definitions for mooring that I've seen before include "Place to keep a boat" , "Place for anchoring or tying up" , "ensuring boat is safe" , "where boat is secured" , "Tying up (a boat)" .)