Medical specialist demanding every substance around emergency room (9)
I believe the answer is:
allergist
'medical specialist' is the definition.
(I know that Allergist is a type of medical specialist)
'every substance around emergency room' is the wordplay.
'every' becomes 'all' (similar in meaning).
'substance' becomes 'gist' (synonyms).
'around' means one lot of letters goes inside another.
'emergency room' becomes 'ER' (abbreviation popularized by US TV series).
'all'+'gist'='allgist'
'allgist' going around 'er' is 'ALLERGIST'.
'demanding' acts as a link.
(Other definitions for allergist that I've seen before include "expert at medical centre?" , "Medic" .)