First number should be zero (6)
I believe the answer is:
nought
'zero' is the definition.
(synonyms)
'first number should be' is the wordplay.
'first' is a charade indicator (letters next to each other).
'number' becomes 'n' (n can represent any number).
'should be' becomes 'ought' (I've seen this before).
'n' next to 'ought' is 'NOUGHT'.
(Other definitions for nought that I've seen before include "Go hunt for nothing" , "The digit 0" , "Love" , "Zero" , "Zilch" .)