Blue boy of flagging significance? (5)

I believe the answer is:

peter

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'blue boy' is the definition.
'Peter' can be an answer for 'boy'. I am unsure of the 'blue' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for peter that I've seen before include "Boy's name - safe" , "Boy's name (maybe blue)" , "Lord -- Wimsey (Sayers)" , "Saint at the Pearly Gates?" , "Safe - name for a man" .)

I've seen this clue in the Evening Standard.

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