Solemn robes altered (5)
I believe the answer is:
sober
'solemn' is the definition.
('sober' can be a synonym of 'solemn')
'robes altered' is the wordplay.
'altered' indicates an anagram.
'robes' with letters rearranged gives 'SOBER'.
(Other definitions for sober that I've seen before include "Temperate" , "Serious; muted in colour" , "Dignified and sombre in manner or character" , "On the wagon" , "Unfuddled" .)