A head teacher's part (4)

I believe the answer is:

each

'a head' is the definition.
(as in a phrase like '50 a head')

'teacher's part' is the wordplay.
'part' says the answer is hidden in the clue.
'EACH' can be found hidden inside 'teachers'.

(Other definitions for each that I've seen before include "every one of a group" , "every one individually" , "every individual" , "Every one separately" , "Per capita, apiece" .)

I've seen this clue in The Telegraph.

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