... regarding a specialist (5)
I believe the answer is:
anent
'regarding' is the definition.
(archaic term meaning about)
'a specialist' is the wordplay.
'a' becomes 'an'.
'specialist' becomes 'ent' (abbreviation of ear nose and throat).
'an'+'ent'='ANENT'
(Other definitions for anent that I've seen before include "About (Scottish)" , "Concerning (archaic)" , "Concerned with" .)