Without one, you can't be a cash customer (4)
I believe the answer is:
bean
'a cash customer' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'without one you can't be' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
'be' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bean that I've seen before include "Kidney-shaped seed as for coffee" , "Nut" , "Seed of a leguminous plant" , "Fava, kidney or mung, say" , "See 12 Down" .)