St Laurent occasionally cleaned out pool (4)

I believe the answer is:

tarn

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'cleaned out pool' is the definition.
'tarn' can be an answer for 'pool' (thesaurus). I'm unsure of the 'cleaned out' bit.

'st laurent occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'st'+'laurent'='stlaurent'
The alternating letters of 'stlaurent' are 'TARN'.

Can you help me to learn more?

(Other definitions for tarn that I've seen before include "Scottish mountain lake" , "Small mountain lake" , "Mountain pool: French department" , "High water" .)

I've seen this clue in The Telegraph.

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