It burns when faculty head steps in for leader in fight (4)
I believe the answer is:
fuel
'it burns when faculty head' is the definition.
The definition suggests a singular noun which matches the answer.
'steps in for leader in fight' is the wordplay.
'steps in' becomes 'uel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' says to put letters next to each other (I've seen this in other clues).
'leader in' indicates taking the first letters.
The first letter of 'fight' is 'f'.
'uel' after 'f' is 'FUEL'.
Can you help me to learn more?
(Other definitions for fuel that I've seen before include "Combustibles" , "coal etc" , "Coal or gas?" , "Eg, coal or petrol" , "Substance to produce energy" .)