Prohibit regular extraction from brain? (3)
I believe the answer is:
ban
'prohibit' is the definition.
(banning is a kind of prohibiting)
'regular extraction from brain?' is the wordplay.
'regular extraction from' indicates alternate letters.
The alternating letters of 'brain' are 'BAN'.
(Other definitions for ban that I've seen before include "Officially prohibit" , "outlaw" , "'Forbid, prohibit (3)'" , "Hungarian governor < to forbid" , "Exile" .)