Old man seen with Irish couple (4)
I believe the answer is:
pair
'couple' is the definition.
(I know that couple can be written as pair)
'old man seen with irish' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'seen with' is a charade indicator (letters next to each other).
'irish' becomes 'IR' (abbreviation).
'pa'+'ir'='PAIR'
(Other definitions for pair that I've seen before include "Two of a kind, as in cards" , "Twins" , "Couple - wed" , "flight of stairs" , "Twosome" .)