A growing source of fuel (4)
I believe the answer is:
tree
'source of fuel' is the definition.
'tree' can be an answer for 'source' (I have seen 'Source of wood ' mean 'tree' so perhaps 'source' could also mean 'tree'). I am not sure about the 'of fuel' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tree that I've seen before include "Beech, for example" , "Theme" , "Diagram representing different relationships" , "Plane perhaps" , "Stand for mugs" .)