Standard upheld by a Druid priestess (5)
I believe the answer is:
norma
'druid priestess' is the definition.
(character in the opera Norma by Vincenzo Bellini)
'standard upheld by a' is the wordplay.
'standard' becomes 'norm' (norm is a kind of standard).
'upheld by' says to put letters next to each other (in a down clue, some letters hold others up).
'norm'+'a'='NORMA'
(Other definitions for norma that I've seen before include "Roman (anag) -- opera" , "Bellini's heroine" , "Female" , "musical work" , "Operatic lady round the manor" .)