Spirited fourth phase of play requires energy (6)
I believe the answer is:
active
'spirited' is the definition.
(thesaurus)
'fourth phase of play requires energy' is the wordplay.
'fourth phase of play' becomes 'Act IV' (fourth act of a play).
'requires' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'activ'+'e'='ACTIVE'
(Other definitions for active that I've seen before include "Dynamic, functioning" , "(Of mind) alert, lively" , "Busy, energetic" , "Moving, doing things" , "militant" .)