Regular fuel found in grass (8)
I believe the answer is:
repeated
'regular' is the definition.
(thesaurus)
'fuel found in grass' is the wordplay.
'fuel' becomes 'peat' (peat is a kind of fuel).
'found in' is an insertion indicator.
'grass' becomes 'reed' (type of grass plant).
'peat' put into 'reed' is 'REPEATED'.
(Other definitions for repeated that I've seen before include "Continual" , "Saying again" , "Iterated" , "Recurring again and again" , "Said again" .)