Nothing's unknown about rejected love (4)
I believe the answer is:
zero
'nothing's' is the definition.
(synonyms)
'unknown about rejected love' is the wordplay.
'unknown' becomes 'z' (algebra).
'about' becomes 're' (regarding).
'rejected' says the letters should be written backwards.
'love' becomes 'o' (love means zero in tennis).
're' reversed gives 'er'.
'z'+'er'+'o'='ZERO'
(Other definitions for zero that I've seen before include "Less than one" , "Nowt" , "Nothing; freezing point (in degrees centigrade)" , "Love" , "Duck" .)