With pure intent, changed, getting thinner (10)
I believe the answer is:
turpentine
'thinner' is the definition.
(turpentine is used to thin paints)
'with pure intent changed' is the wordplay.
'with' means one lot of letters go next to another.
'changed' indicates an anagram.
'pure' next to 'intent' is 'pureintent'.
'pureintent' with letters rearranged gives 'TURPENTINE'.
'getting' is the link.
(Other definitions for turpentine that I've seen before include "Oil used in mixing paints" , "Paint/varnish spirit" , "Oil used in varnishes" , "Thick, oily resin used as paint thinner" , "artist's stuff" .)