An Irish saint and an excellent fellow (5)
I believe the answer is:
aidan
'an irish saint' is the definition.
(I've seen this in another clue)
'an excellent fellow' is the wordplay.
'an excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'fellow' becomes 'Dan' (man's name).
'ai'+'dan'='AIDAN'
'and' acts as a link.
(Other definitions for aidan that I've seen before include "Saint associated with Lindisfarne" , "Irish fellow" .)