Black stuff's covering northern lakes (5)

I believe the answer is:

tarns

'lakes' is the definition.
(I know this)

'black stuff's covering northern' is the wordplay.
'black stuff's' becomes 'tar's' (tar is a black substance).
'covering' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'tars' going around 'n' is 'TARNS'.

(Another definition for tarns that I've seen is " Mountain lakes".)

I've seen this clue in The Telegraph.

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