Constant fuel source covered by grass (8)
I believe the answer is:
repeated
'constant' is the definition.
(thesaurus)
'fuel source covered by grass' is the wordplay.
'fuel source' becomes 'peat' (I've seen this in another clue).
'covered by' means one lot of letters goes inside another.
'grass' becomes 'reed' (type of grass plant).
'peat' placed inside 'reed' is 'REPEATED'.
(Other definitions for repeated that I've seen before include "Said again" , "Iterated" , "Saying again" , "Recurring again and again" , "Continual" .)