Relation's pleasant embracing sweetheart (5)
I believe the answer is:
niece
'relation's' is the definition.
(niece is a kind of relation)
'pleasant embracing sweetheart' is the wordplay.
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'embracing' indicates putting letters inside.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'nice' placed around 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "Family girl" , "Female family member" , "[PRIEST'S] daughter?" , "One of the family" , "Family relation" .)