During flight, energy may reduce (7)
I believe the answer is:
lighten
'reduce' is the definition.
(thesaurus)
'during flight energy may' is the wordplay.
'during' indicates the answer is hidden within the clue.
'LIGHTEN' is hidden in the letters of 'flight energy may'.
(Other definitions for lighten that I've seen before include "lift" , "Make paler" , "Relieve" , "Reduce load" , "Make less burdensome" .)