Block entrance to office used by surgeon (4)
I believe the answer is:
veto
'block' is the definition.
(thesaurus)
'entrance to office used by surgeon' is the wordplay.
'entrance to' says to take the initial letters.
'used by' means one lot of letters go next to another.
'surgeon' becomes 'vet' (I've seen this in other clues).
The initial letter of 'office' is 'o'.
'o' put after 'vet' is 'VETO'.
(Other definitions for veto that I've seen before include "Authoritative prohibition" , "Refusal to assent" , "Right to prevent proposed action" , "black ball" , "bar" .)