Saintly Irishman, a first-class fellow (5)
I believe the answer is:
aidan
'saintly irishman' is the definition.
The definition suggests a singular noun which matches the answer.
'a first-class fellow' is the wordplay.
'a first class' becomes 'ai' (A1 - top quality).
'fellow' becomes 'dan' (man's name).
'ai'+'dan'='AIDAN'
Can you help me to learn more?
(Other definitions for aidan that I've seen before include "Saint associated with Lindisfarne" , "Irish fellow" .)