Some teachers selected a head (4)
I believe the answer is:
each
'a head' is the definition.
(I've seen this before)
'some teachers selected' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'EACH' is hidden within 'teachers selected'.
(Other definitions for each that I've seen before include "...per head" , "every one individually" , "every individual" , "This way for a place" , "Every one of a number" .)