Maintain you said acidity to be a thing of the past (6)
I believe the answer is:
uphold
'maintain' is the definition.
(I know that maintain can be written as uphold)
'you said acidity to be a thing of the past' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'acidity' becomes 'ph'.
'to be a thing of the past' becomes 'old' (I can't explain this - if you can you should believe this answer much more).
'you' is a homophone of 'u'.
'u'+'ph'+'old'='UPHOLD'
Can you help me to learn more?
(Other definitions for uphold that I've seen before include "'Support, a court decision, say (6)'" , "Of appeal court, endorse earlier decision" , "Champion" , "Support, maintain" , "'Support, the law say (6)'" .)