End of equation should = 0 (6)
I believe the answer is:
nought
'0' is the definition.
(synonyms)
'end of equation should' is the wordplay.
'end of' says to take the final letters.
'should' becomes 'ought' (synonyms).
The last letter of 'equation' is 'n'.
'n'+'ought'='NOUGHT'
(Other definitions for nought that I've seen before include "Love" , "Nil" , "Poetically nothing" , "Go hunt for nothing" , "Zilch" .)