Relative back in EC energised at first (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'back in ec energised at first' is the wordplay.
'back' is a reversal indicator.
'at first' suggests taking the first letters.
The first letter of 'energised' is 'e'.
'in' back-to-front is 'ni'.
'ni'+'ec'+'e'='NIECE'
(Other definitions for niece that I've seen before include "member of different generation" , "Brother's daughter" , "Family girl" , "girl further down the line" , "Young female relative" .)