Age of occasionally frail writer (5)
I believe the answer is:
ripen
'age' is the definition.
(I've seen this in another clue)
'occasionally frail writer' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'writer' becomes 'pen' (something which writes).
The alternate letters of 'frail' are 'ri'.
'ri'+'pen'='RIPEN'
'of' is the link.
(Other definitions for ripen that I've seen before include "Get ready to eat" , "Develop fully" , "Become ready for eating" , "Get ready to open" , "Become ready for picking" .)