Retired relatives keeping energetic (5)
I believe the answer is:
vital
'energetic' is the definition.
('vital' can be a synonym of 'energetic')
'retired relatives keeping' is the wordplay.
'retired' says the letters should be written backwards.
'keeping' says the answer is hidden in the clue.
'lativ' can be found hidden inside 'relatives'.
'lativ' back-to-front is 'VITAL'.
(Other definitions for vital that I've seen before include "Essential; lively" , "Critical; life-sustaining" , "Desperately important" , "Urgently needed or absolutely necessary" , "Of key importance" .)