Final bill up on energy … perfect (10)
I believe the answer is:
infallible
'perfect' is the definition.
(I've seen this before)
'final bill up on energy' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'on' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'final'+'bill'='finalbill'
'finalbill' anagrammed gives 'infallibl'.
'infallibl'+'e'='INFALLIBLE'
(Other definitions for infallible that I've seen before include "Always working" , "faultless" , "Always right" , "Incapable of error" , "Perfect" .)