Like Al, a boy in blue (3)

I believe the answer is:

roy

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

'boy in blue' is the definition.
'Roy' can be an answer for 'boy'. I am not sure about the 'in blue' bit.

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

Can you help me to learn more?

(Other definitions for roy that I've seen before include "'Rob ..., ... Rogers (3)'" , "Rob --, Scots outlaw" , "Man" , "Rob . . . was a notorious Scottish outlaw" , "-- Lichtenstein, US artist" .)

I've seen this clue in the Evening Standard.

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