Heavy charge lit to explode (9)
I believe the answer is:
lethargic
'heavy' is the definition.
(thesaurus)
'charge lit to explode' is the wordplay.
'to explode' is an anagram indicator.
'charge'+'lit'='chargelit'
'chargelit' is an anagram of 'LETHARGIC'.
(Other definitions for lethargic that I've seen before include "Lacking in energy or vitality" , "languid" , "Enervated" , "Sluggish and apathetic" , "without energy" .)