Increased limits for acidity in painkiller (8)

I believe the answer is:

morphine

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'painkiller' is the definition.
(I know that morphine is a type of painkiller)

'increased limits for acidity in' is the wordplay.
'increased' becomes 'more'.
'limits' is an insertion indicator (limit can mean to contain or block off).
'for acidity' becomes 'ph' (I am not sure about this - if you are sure you should believe this answer much more).
'ph'+'in'='phin'
'more' enclosing 'phin' is 'MORPHINE'.

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(Other definitions for morphine that I've seen before include "Pain-easing opiate" , "Opiate for pain relief" , "Powerful pain-reliever" , "Sedative derived from opium" , "Opium drug for relieving pain" .)

I've seen this clue in the Sydney Morning Herald.

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