That’s 1,951 years ago, boy (3)

I believe the answer is:

lad

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'boy' is the definition.
(lad is a kind of boy)

I don't understand how the rest of the clue works.

(Other definitions for lad that I've seen before include "(Housman's Shropshire) boy" , "A young chap into ballads" , "Scottish lover" , "Shaver" , "Chap, fellow" .)

I've seen this clue in the Evening Standard.

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