To a doctor a hospital atmosphere is material (6)
I believe the answer is:
mohair
'is material' is the definition.
(I've seen this before)
'to a doctor a hospital atmosphere' is the wordplay.
'to a doctor' becomes 'mo' (medical officer).
'a hospital' becomes 'h' (abbreviation for hospital on maps).
'atmosphere' becomes 'air' (synonyms).
'mo'+'h'+'air'='MOHAIR'
(Other definitions for mohair that I've seen before include "Fine hair fabric" , "Fabric made from fine silky wool of goat" , "Fine silky wool of the angora goat" , "coat required" , "Fine silky coat of angora goat" .)