Answer seen in favourite fuel (4)
I believe the answer is:
peat
'fuel' is the definition.
(peat is a kind of fuel)
'answer seen in favourite' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'seen in' indicates putting letters inside.
'favourite' becomes 'pet' (synonyms).
'a' going within 'pet' is 'PEAT'.
(Other definitions for peat that I've seen before include "Form of fuel" , "Tape (anag.)" , "Fuel or compost (but no longer to be used as such!)" , "Partially carbonised vegetable matter used as a fuel when dried" , "Dried plant material used as fuel" .)