They're acid, and may be solemn (6)
I believe the answer is:
lemons
'they're acid' is the definition.
The definition suggests an adverb but the answer is not.
'may be solemn' is the wordplay.
'may be' is an anagram indicator.
'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 "Citrus friuts" , "Strangely melons could be citrus fruits" , "Could strange melons be citrus?" .)