A change of attire for number in opera (7)
I believe the answer is:
arietta
'in opera' is the definition.
(I know that arietta is part of opera)
'a change of attire' is the wordplay.
'change of' indicates an anagram (the letter order is changed).
'attire' is an anagram of 'rietta'.
'a'+'rietta'='ARIETTA'
'for number' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for arietta that I've seen before include "Little song" , "Little aria" , "number from opera" , "Short operatic number" , "Short operatic air" .)