Newton should love (6)
I believe the answer is:
nought
'love' is the definition.
(I've seen this before)
'newton should' is the wordplay.
'newton' becomes 'N' (synonyms).
'should' becomes 'ought' (synonyms).
'n'+'ought'='NOUGHT'
(Other definitions for nought that I've seen before include "The digit 0" , "The figure 0" , "Love" , "Zero" , "Poetically nothing" .)