Relative, kind to conserve energy (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'kind to conserve energy' is the wordplay.
'kind' becomes 'nice' (I've seen this before).
'to conserve' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'nice' enclosing 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "Son's cousin" , "A sibling's daughter" , "Young female relative" , "Daughter of a sibling" , "Female family member" .)