Energy can't improve play (5)
I believe the answer is:
enact
'play' is the definition.
(enacting is a kind of playing)
'energy can't improve' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'improve' is an anagram indicator.
'cant' anagrammed gives 'nact'.
'e'+'nact'='ENACT'
(Other definitions for enact that I've seen before include "Decree" , "get into law" , "Play one's part" , "Present" , "Bring in" .)