Error on set of web pages, we’re told (9)
I believe the answer is:
oversight
'error' is the definition.
(oversight is a kind of error)
'on set of web pages we're told' is the wordplay.
'on' becomes 'over' (I've seen this in other clues).
'set of web pages' becomes 'site' (as in a website).
'we're told' shows a homophone (sound like).
'site' sounds like 'sight'.
'over'+'sight'='OVERSIGHT'
(Other definitions for oversight that I've seen before include "what supervisor has" , "Failure to notice - blunder" , "Failure to notice - supervision" , "Inadvertent mistake" , "Management" .)