Irish saint's name attached to opera (5)
I believe the answer is:
aidan
'irish saint's' is the definition.
(I've seen this in another clue)
'name attached to opera' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'attached to' says to put letters next to each other.
'opera' becomes 'Aida' (Aida is an opera by Giuseppe Verdi).
'n' put after 'aida' is 'AIDAN'.
(Other definitions for aidan that I've seen before include "Irish fellow" , "Saint associated with Lindisfarne" .)