Crossword Genius

Lacking siblings and just twelve months old (4,3)

Ross

I believe the answer is:

only one

I'm a little stuck... Click here to teach me more about this clue!

'twelve months old' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'lacking siblings and just' is the wordplay.
'lacking siblings' becomes 'only' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'just' becomes 'one' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'only'+'one'='ONLY ONE'

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I've seen this clue in The Globe and Mail.
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