Retail supplier is Spooner's sole caller (10)
I believe the answer is:
wholesaler
'retail supplier' is the definition.
'wholesaler' can be an answer for 'supplier' (wholesaler is a kind of supplier). I'm not certain of the 'retail' bit.
'spooner's sole caller' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'sole' is 'oles' which is within the answer.
The remaining letters 'whaler' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for wholesaler that I've seen before include "firm that supplies in bulk" , "Bulk trader" , "One supplying retailers in bulk" , "retailer of large quantities" , "Retailer's supplier" .)