Crossword Genius

Swim poorly, swallowing tons — idiot! (8)

Ross

I believe the answer is:

dipstick

'idiot' is the definition.
(informal term for a stupid person)

'swim poorly swallowing tons' is the wordplay.
'swim' becomes 'dip' (dip can mean a brief swim).
'poorly' becomes 'sick' (similar in meaning).
'swallowing' means one lot of letters goes inside another.
'tons' becomes 't'.
'sick' enclosing 't' is 'stick'.
'dip'+'stick'='DIPSTICK'

(Other definitions for dipstick that I've seen before include "Device for checking oil level" , "Turkey" , "plunged into engine oil?" , "Engine oil measurer" , "Rod used to measure oil level in car" .)

I've seen this clue in The Times.
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