Relative's pleasant, full of energy (5)
I believe the answer is:
niece
'relative's' is the definition.
(niece is a kind of relative)
'pleasant full of energy' is the wordplay.
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'full of' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'nice' going around 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "Son's cousin" , "A younger female relative" , "A sibling's child" , "Family girl" , "Brother's daughter" .)