Customer exactly right and not coming back? (6)
I believe the answer is:
patron
'customer' is the definition.
(a customer of a business or service)
'exactly right and not coming back?' is the wordplay.
'exactly right' becomes 'pat' (pat can mean perfectly memorised).
'and not' becomes 'nor' ('nor' means 'and also not').
'coming back?' is a reversal indicator.
'nor' in reverse letter order is 'ron'.
'pat'+'ron'='PATRON'
(Other definitions for patron that I've seen before include "one of the regular customers" , "With help, he" , "Pot ran out for protector" , "Patrick for Ireland, for example" , "Sponsor or supporter as of the arts" .)