Behind schoolmaster, necking drinks (6)
I believe the answer is:
astern
'behind' is the definition.
(at the back or behind of a ship)
'schoolmaster necking drinks' is the wordplay.
'drinks' indicates the answer is hidden within the clue (words in the clue have absorbed the answer).
'ASTERN' is hidden in the letters of 'schoolmaster necking'.
(Other definitions for astern that I've seen before include "Towards the rear of a ship" , "(In relation to ship) behind" , "Abaft" , "Rearward" , "following behind vessel" .)