Should start with nine, or half a dozen (3)
I believe the answer is:
six
'half a dozen' is the definition.
(synonyms)
'should start with nine' is the wordplay.
'start with' suggests taking the first letters.
'nine' becomes 'IX' (roman numeral).
The first letter of 'should' is 's'.
's'+'ix'='SIX'
'or' is the link.
(Other definitions for six that I've seen before include "VI" , "Airborne boundary hit" , "Number of bowls in a cricket over" , "Half a dozen" , "Knock for . . . - surprise or defeat" .)