Doctor upholds firm means of treating shock (4)
I believe the answer is:
comb
'means of treating shock' is the definition.
(a 'shock' can mean a thick mass of hair)
'doctor upholds firm' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'upholds' means one lot of letters go next to another (some letters go underneath or uphold others).
'firm' becomes 'co' (a firm is a company).
'mb' put after 'co' is 'COMB'.
(Other definitions for comb that I've seen before include "Search methodically" , "Rummage through" , "Go over" , "Systematically search" , "Device for tidying one's appearance" .)