Swim poorly, swallowing tons — idiot! (8)
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" .)