Exhaust school writer on the day (5)
I believe the answer is:
spend
'exhaust' is the definition.
(I know that exhaust is a kind of spend)
'school writer on the day' is the wordplay.
'school' becomes 'S'.
'writer' becomes 'pen' (something which writes).
'on' is a charade indicator (letters next to each other).
'the day' becomes 'd' (abbreviation for day).
's'+'pen'+'d'='SPEND'
(Other definitions for spend that I've seen before include "amount" , "Run through" , "Pay out or use up" , "Lay out money" , "Outlay" .)