Temptation to retain energy is leading to relaxation (7)
I believe the answer is:
leisure
'to relaxation' is the definition.
(leisure is a kind of relaxation)
'temptation to retain energy is leading' is the wordplay.
'temptation' becomes 'lure' (lure is a kind of temptation).
'to retain' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mcu).
'leading' means one lot of letters go next to another.
'e'+'is'='eis'
'lure' placed around 'eis' is 'LEISURE'.
(Other definitions for leisure that I've seen before include "Rest, relaxation" , "may suggest fun and games" , "Rue lies about rest period" , "Time away from work" , "Ease; time off work" .)