Without energy, a devil is hard to reprove (8)
I believe the answer is:
admonish
'to reprove' is the definition.
('admonish' can be a synonym of 'reprove')
'without energy a devil is hard' is the wordplay.
'without' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mcs).
'devil' becomes 'demon' (synonyms).
'is hard' becomes 'ish' (I can't explain this - if you can you should believe this answer much more).
'a'+'demon'='ademon'
'ademon' with 'e' taken out is 'admon'.
'admon'+'ish'='ADMONISH'
(Other definitions for admonish that I've seen before include "criticise" , "Warn sternly" , "Chide or reprimand" , "Reprove, warn" , "Tell off" .)