A Conservative I defied verbally’s become sour (9)
I believe the answer is:
acidified
'a conservative i' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally define each other.
'defied verbally's become sour' is the wordplay.
I cannot quite understand how this works, but
'sour' could be 'acid' (I've seen this before) and 'acid' is located in the answer.
This explanation may well be incorrect...
(Another definition for acidified that I've seen is " Made into something turning litmus red".)