Single month — November or August? (6)
I believe the answer is:
solemn
'august?' is the definition.
(thesaurus)
'single month november' is the wordplay.
'single' becomes 'sole' (synonyms).
'month' becomes 'M' (standard abbreviation).
'november' becomes 'N' (phonetic alphabet: alpha, bravo, charlie etc.).
'sole'+'m'+'n'='SOLEMN'
'or' acts as a link.
(Other definitions for solemn that I've seen before include "Grave and serious about melons" , "Formal, ceremonius" , "Portentous" , "'Sedate, grave (6)'" , "Marked by pomp or ceremony" .)