One able to retrace his line of business? (7)
I believe the answer is:
caterer
'his line of business?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one able to retrace' is the wordplay.
'retrace' anagrams to 'CATERER'.
Though, I am not sure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for caterer that I've seen before include "Person who supplies food for social events" , "One providing food and drink" , "Someone who provides food and service" , "Provider of food/drink" , "Purveyor" .)