They’re acid and possibly solemn (6)
I believe the answer is:
lemons
'they're acid' is the definition.
The definition suggests an adverb but the answer is not.
'possibly solemn' is the wordplay.
'possibly' indicates anagramming the letters.
'solemn' is an anagram of 'LEMONS'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for lemons that I've seen before include "Strangely melons could be citrus fruits" , "Could strange melons be citrus?" , "Citrus friuts" .)