Headmaster necessitates clutching behind (6)
I believe the answer is:
astern
'behind' is the definition.
(at the back or behind of a ship)
'headmaster necessitates clutching' is the wordplay.
'clutching' says the answer is hidden in the clue.
'ASTERN' is hidden within 'headmaster necessitates'.
(Other definitions for astern that I've seen before include "Towards rear of a vessel" , "Behind (a boat)" , "(In relation to ship) behind" , "Nautically behind" , "Abaft" .)