Customer in Celt's novel (6)
I believe the answer is:
client
'customer' is the definition.
(synonyms)
'in celt's novel' is the wordplay.
'novel' indicates an anagram (letters in a new or novel order).
'in'+'celt'='incelt'
'incelt' with letters rearranged gives 'CLIENT'.
(Other definitions for client that I've seen before include "Employer of professional" , "Customer - dependant" , "Computer program" , "Dependant, hanger-on" , "Person using professional services" .)