Relative, friendly, full of energy (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'friendly full of energy' is the wordplay.
'friendly' becomes 'nice'.
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'nice' enclosing 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "member of different generation" , "Family relative" , "Brother's girl" , "A sibling's child" , "[PRIEST'S] daughter?" .)