Make a mistake with child who's been trusted to fetch something (6,3)
I believe the answer is:
errand boy
'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" .)