Critic unreasonably acid (6)
I believe the answer is:
citric
'acid' is the definition.
(citric acid is a kind of acid)
'critic unreasonably' is the wordplay.
'unreasonably' is an anagram indicator (I've seen this in other clues).
'critic' with letters rearranged gives 'CITRIC'.
(Other definitions for citric that I've seen before include "Lemony" , "Fruity" , "Derived from eg, lemon" , "Derived from, eg, lemons" , "Acid, from lemons say" .)