Amount of heat head of Harrow used during school session (5)
I believe the answer is:
therm
'amount of heat' is the definition.
(I've seen this before)
'head of harrow used during school session' is the wordplay.
'head of' says to take the initial letters.
'used during' indicates putting letters inside.
'school session' becomes 'term' (I've seen this before).
The initial letter of 'harrow' is 'h'.
'h' inserted into 'term' is 'THERM'.
(Other definitions for therm that I've seen before include "Unit of measurement of heat" , "Amount of heat" , "Unit of heat used in the UK as the basis of gas bills" , "Indicator of heat" , "Unit of heat and gas measure" .)