Only second half of hymn is formal and dignified (6)
I believe the answer is:
solemn
'formal and dignified' is the definition.
(solemn means highly formal and serious)
'only second half of hymn' is the wordplay.
'only' becomes 'sole' (synonyms).
'second half of' suggests halving the letters (the back or second half of the word).
'hymn' halved is 'mn'.
'sole'+'mn'='SOLEMN'
'is' is the link.
(Other definitions for solemn that I've seen before include "gloomy" , "Serious, dignified or ceremonial" , "Grave and serious about melons" , "Portentous" , "Marked by pomp or ceremony" .)