Professional line receiving treatment (5,8)
I believe the answer is:
civil engineer
'professional' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'line receiving treatment' is the wordplay.
'treatment' is an anagram indicator (letters treated or processed differently).
'line'+'receiving'='linereceiving'
'linereceiving' with letters rearranged gives 'CIVIL ENGINEER'.
Can you help me to learn more?
(Other definitions for civil engineer that I've seen before include "Eg, road or bridge designer" , "Brunel, e.g." .)