Answer found in favourite fuel (4)
I believe the answer is:
peat
'fuel' is the definition.
(peat is a kind of fuel)
'answer found in favourite' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'found in' indicates putting letters inside.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'a' placed into 'pet' is 'PEAT'.
(Other definitions for peat that I've seen before include "Form of fuel" , "Natural solid fuel" , "Tape (anag.)" , "Fuel from boggy land" , "Bog material" .)