It's said to make the heart grow fonder (7)
I believe the answer is:
absence
'it's said to make the heart grow fonder' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for absence that I've seen before include "form of inattention?" , "Truancy" , "It may make the heart grow fonder" , "It's said to make the heart grow fonder" , "reason for better relationship?" .)