Bog buildup (4)
I believe the answer is:
peat
'bog buildup' 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 "It's used for fuel and in the garden" , "Natural solid fuel" , "Tape (anag.)" , "Fuel from boggy land" , "Partially carbonised vegetable matter saturated with water" .)