Energy, at the present time, no longer sufficient (4)
I believe the answer is:
enow
'no longer sufficient' is the definition.
(archaic version of 'enough')
'energy at the present time' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'at' means one lot of letters go next to another.
'the present time' becomes 'now' (now means at the present time).
'e'+'now'='ENOW'
(Other definitions for enow that I've seen before include "adequate" , "up in Scotland" , "Enough (archaic)" , "Old enough" , "a moment ago" .)