One's always right to cover rear end with swimming costume (8)
I believe the answer is:
customer
'one's' is the definition.
(thesaurus)
'rear end with swimming costume' is the wordplay.
'end' indicates one should take the final letters.
'with' is a charade indicator (letters next to each other).
'swimming' indicates an anagram (letters swim around into a new order).
The final letter of 'rear' is 'r'.
'costume' anagrammed gives 'custome'.
'r' after 'custome' is 'CUSTOMER'.
'always right to cover' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for customer that I've seen before include "Someone who pays for goods or services" , "The person who is said in business to be always right" , "Buyer of goods or services" , "Shopper (or now rail passenger!)" , "Client (who's always right?)" .)