A fraction that is separate (5)
I believe the answer is:
apart
'is separate' is the definition.
(similar in meaning)
'a fraction' is the wordplay.
'fraction' becomes 'part' (I've seen this in another clue).
'a'+'part'='APART'
'that' is the link.
(Other definitions for apart that I've seen before include "In private" , "Isolated" , "No longer living together" , "in isolation" , "cut off from others" .)