Separated and sure to be reunited (7)
I believe the answer is:
asunder
'separated' is the definition.
(I've seen this in another clue)
'and sure to be reunited' is the wordplay.
'to be reunited' indicates anagramming the letters (I've seen 'reuniting' mean this (letters are put together again in a new way)).
'and'+'sure'='andsure'
'andsure' with letters rearranged gives 'ASUNDER'.
(Other definitions for asunder that I've seen before include "Sad rune (anag.)" , "Apart - in pieces" , "bits?" , "going to pieces" , "Let no man put -" .)