Riddle for each partner embracing sweetheart (8)
I believe the answer is:
permeate
'riddle' is the definition.
(thesaurus)
'each partner embracing sweetheart' is the wordplay.
'each' becomes 'per'.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'embracing' is an insertion indicator.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'mate' going around 'e' is 'meate'.
'per'+'meate'='PERMEATE'
'for' acts as a link.
(Other definitions for permeate that I've seen before include "Suffuse" , "Seep through" , "Pass through" , "Flow or spread into every part of" , "Spread or diffuse through" .)