Age of retired Irish writer (5)
I believe the answer is:
ripen
'age' is the definition.
(I've seen this in another clue)
'retired irish writer' is the wordplay.
'retired' says the letters should be written in reverse.
'irish' becomes 'IR' (abbreviation for Irish).
'writer' becomes 'pen' (something which writes).
'ir' reversed gives 'ri'.
'ri'+'pen'='RIPEN'
'of' is the link.
(Other definitions for ripen that I've seen before include "Make ready" , "Mature, age" , "come near to falling" , "Grow older" , "Come to maturity like fruit" .)