Organic fuel (4)
I believe the answer is:
'organic fuel' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for peat that I've seen before include "Bog vegetable matter, turf" , "Vegetable deposit used as fuel and fertiliser" , "Fuel dug from bog" , "Natural solid fuel" , "Dried turf" .)