American trunks are large to cover swimmer's rear – most vendors here are out of gear (3,4,4)
I believe the answer is:
car boot sale
'cover swimmer's rear most vendors here are out of gear' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'american trunks are large' is the wordplay.
I cannot really understand how this works, but
'american' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is present in the answer.
'trunks' could be 'boots' ('boot' can be a synonym of 'trunk') and 'boots' is located in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for car boot sale that I've seen before include "Where you'll see many a bumper" , "Vending from back of private vehicles" , "place for bargains?" , "Vending of second-hand goods from private vehicles" , "field event?" .)