Reduced settlement, not entirely ample, regarding acid (6)
I believe the answer is:
citric
'regarding acid' is the definition.
'citric' can be an answer for 'acid' (I have seen 'Acid of lemons ' mean 'citric' so perhaps 'acid' could also mean 'citric'). I am not certain of the 'regarding' bit.
'reduced settlement not entirely ample' is the wordplay.
'reduced settlement' becomes 'cit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'not entirely' means to remove the last letter (not all of the word).
'ample' becomes 'rich' (synonyms).
'rich' with its final letter taken away is 'ric'.
'cit'+'ric'='CITRIC'
Can you help me to learn more?
(Other definitions for citric that I've seen before include "Lemony" , "Derived from, eg, lemons" , "Derived from eg, lemon" , "Fruity" , "Sort of acid" .)