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' means one lot of letters goes inside another.
'favourite' becomes 'pet' (synonyms).
'a' inserted inside '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" .)