To the point where opiates failed to contain onset of pain (8)
I believe the answer is:
apposite
'to the point' is the definition.
(thesaurus)
'opiates failed to contain onset of pain' is the wordplay.
'failed' indicates anagramming the letters.
'to contain' indicates putting letters inside.
'onset of' says to take the initial letters.
The initial letter of 'pain' is 'p'.
'opiates' anagrammed gives 'aposite'.
'aposite' enclosing 'p' is 'APPOSITE'.
'where' acts as a link.
(Other definitions for apposite that I've seen before include "Suited for the purpose" , "A pit pose could be to the purpose" , "Appropriate, apt" , "Appropriate for purpose or occasion" , "Germane" .)