Left one opening to tie up (6)
I believe the answer is:
ligate
'tie up' is the definition.
(I've seen this in another clue)
'left one opening' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'opening' could be 'gate' (a gate is an opening or entrance) and 'gate' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Another definition for ligate that I've seen is " Tie".)