Growing source of fuel (5)
I believe the answer is:
trees
'growing' is the definition.
Although both the answer and definition are verbs, they are not in the same form of the verb.
'source of fuel' is the wordplay.
'source' becomes 'tree' (I have seen 'Source of wood ' mean 'tree' so perhaps 'source' could also mean 'tree').
'of fuel' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'tree'+'s'='TREES'
Can you help me to learn more?
(Other definitions for trees that I've seen before include "Large plants" , "''I see ... of green / Red roses too'' (What a Wonderful World)" , "Steer into the plants" , "Long lock of hair" , "Components of forest" .)