The cross one is Roman (6)
I believe the answer is:
eleven
This could be a double definition.
'the cross one' is the first definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'roman' is the second definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for eleven that I've seen before include "'Legs' bingo number" , "Football or cricket team?" , "Smallest positive three-syllable integer" , "Age to start secondary education" , "Number, on a team maybe" .)