Relation's sweet embracing sweetheart (5)
I believe the answer is:
niece
'relation's' is the definition.
(niece is a kind of relation)
'sweet embracing sweetheart' is the wordplay.
'sweet' becomes 'nice' (I've seen this in other clues).
'embracing' is an insertion indicator.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'nice' going around 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "[PRIEST'S] daughter?" , "A sibling's daughter" , "Brother's girl" , "Criminal" , "Family relative" .)