Plan part of Christmas term in detail (10)
I believe the answer is:
mastermind
'plan' is the definition.
(I know that mastermind is a more specific form of plan)
'part of christmas term in detail' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'MASTERMIND' is hidden within 'christmas term in detail'.
(Other definitions for mastermind that I've seen before include "Person conceiving a project - TV programme" , "Ringleader" , "Criminal planner" , "Be behind" , "Intellectual" .)