Crossword Genius

Make a mistake with child who's been trusted to fetch something (6,3)

Ross

I believe the answer is:

errand boy

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'been trusted to fetch something' is the definition.
I can't judge whether this definition defines the answer.

'make a mistake with child' is the wordplay.
'make a mistake' becomes 'err' (to err is to make an error).
'with' becomes 'and'.
'child' becomes 'boy' (boy is a kind of child).
'err'+'and'+'boy'='ERRAND BOY'

'who's' is the link.

Can you help me to learn more?

(Other definitions for errand boy that I've seen before include "Young assistant" , "One making, eg, deliveries" , "Young runner" , "who gets the greens?" , "Young messenger" .)

I've seen this clue in The Irish Times.
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