He serves member in prohibition (6)
I believe the answer is:
barman
'he serves' is the definition.
(I've seen this before)
'member in prohibition' is the wordplay.
'member' becomes 'arm' (member can mean a limb).
'in' indicates putting letters inside.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'arm' going into 'ban' is 'BARMAN'.
(Other definitions for barman that I've seen before include "Drinks server" , "Male who serves in a pub" , "he might be responsible for you getting shot" , "He serves drink" , "Club worker" .)