Some retrospectively secure MOT success for client (8)
I believe the answer is:
customer
'for client' is the definition.
(synonyms)
'some retrospectively secure mot success' is the wordplay.
'retrospectively' says the letters should be written backwards.
'some' indicates a hidden word (some parts of the word are used).
'remotsuc' is hidden within 'secure mot success'.
'remotsuc' backwards is 'CUSTOMER'.
(Other definitions for customer that I've seen before include "Person shopping" , "Person buying" , "Shop client" , "Client (who's always right?)" , "Buyer of goods or services" .)