Thus restricting doctor, a ban (7)
I believe the answer is:
embargo
'ban' is the definition.
(thesaurus)
'thus restricting doctor a' is the wordplay.
'thus' becomes 'ergo' (I've seen this before).
'restricting' means one lot of letters goes inside another.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'mb'+'a'='mba'
'ergo' enclosing 'mba' is 'EMBARGO'.
(Other definitions for embargo that I've seen before include "Prohibit" , "Trade stoppage" , "Ban, veto" , "restrict trade" , "Order forbidding ships to enter or leave port, and a ban on trade" .)