Scot isn't able to cover first two of Ullapool drains (8)

I believe the answer is:

cannulae

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'drains' is the definition.
(plural of cannula - a cannula drains liquid)

'scot isn't able to cover first two of' is the wordplay.
'scot isn't able to' becomes 'cannae' (Scottish dialect form of 'cannot').
'cover' means one lot of letters goes inside another.
'first two of' becomes 'ul' (I can't explain this - if you can you should believe this answer much more).
'cannae' going around 'ul' is 'CANNULAE'.

'ullapool' is the link.
I am not very happy about this link. It may belong to another bit of the clue.

Can you help me to learn more?

I've seen this clue in The Guardian.

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