Priestess relaxes after admitting principally tender relative (10)
I believe the answer is:
stepsister
'priestess relaxes after admitting principally' is the definition.
I can't tell whether this definition defines the answer.
'tender relative' is the wordplay.
'tender' becomes 'step' (I can't justify this - if you can you should believe this answer much more).
'relative' becomes 'sister' (sister is a kind of relative).
'step'+'sister'='STEPSISTER'
Can you help me to learn more?
(Other definitions for stepsister that I've seen before include "one who hated Cinderella" , "Relation gained through a second marriage" , "member of the family" , "relative in law" .)