A boy, ten, helped out? That's totally unacceptable (6,3,4)
I believe the answer is:
beyond the pale
'totally unacceptable' is the definition.
(I've seen this before)
'a boy ten helped out?' is the wordplay.
'out?' is an anagram indicator (out can mean wrong or inaccurate).
'a'+'boy'+'ten'+'helped'='aboytenhelped'
'aboytenhelped' with letters rearranged gives 'BEYOND THE PALE'.
'that's' acts as a link.
(Other definitions for beyond the pale that I've seen before include "Not understood by untanned people" , "completely washed out?" , "Unacceptable (behaviour)" , "Morally or socially intolerable" , "Socially unacceptable" .)