Crossword Genius

A matching pair, boy and his namesake less alike ultimately (7)

Ross

I believe the answer is:

jimjams

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

'namesake less alike ultimately' is the definition.
I can't tell whether this defines the answer.

'a matching pair boy and his' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'boy' could be 'jim' and 'jim' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for jimjams that I've seen is " Child's nightclothes".)

I've seen this clue in The Times.
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