Very hot, that is, in food cooked in fat (5)
I believe the answer is:
fiery
'very hot that is in food cooked in fat' is the definition.
'fiery' can be an answer for 'very hot' (I've seen this in another clue). I am unsure of the rest of the definition.
'very hot that is in' is the wordplay.
'very hot' becomes 'fry' (I can't justify this - if you can you should believe this answer much more).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'in' indicates putting letters inside.
'fry' going around 'ie' is 'FIERY'.
Can you help me to learn more?
(Other definitions for fiery that I've seen before include "Impetuous; hot-tasting" , "irascible" , "Ardent" , "Intensely hot" , "Explosive (e.g. temper)" .)