Client worried Sue and Tom, being in credit (8)
I believe the answer is:
customer
'client' is the definition.
('customer' can be a synonym of 'client')
'worried sue and tom being in credit' is the wordplay.
'worried' indicates anagramming the letters.
'and' says to put letters next to each other.
'being in' indicates putting letters inside.
'credit' becomes 'cr' (**).
'sue'+'tom'='suetom'
'suetom' with letters rearranged gives 'ustome'.
'ustome' going within 'cr' is 'CUSTOMER'.
(Other definitions for customer that I've seen before include "Shopper or client - always right" , "One who buys goods or services" , "Shop client" , "Selfridge said one is always right" , "Buyer of goods or services" .)