Key specialist treated pain first (7)
I believe the answer is:
pianist
'key specialist' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'treated pain first' is the wordplay.
'treated' indicates an anagram.
'first' becomes 'ist' (ist resembles 1st).
'pain' is an anagram of 'pian'.
'pian'+'ist'='PIANIST'
Can you help me to learn more?
(Other definitions for pianist that I've seen before include "Key expert" , "Musician" , "Ashkenazy, say" , "Key worker," , "Key player" .)