Past his prime, he may still be useful in the kitchen (3,5)
I believe the answer is:
old timer
'prime he may still be useful in the kitchen' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'past his' is the wordplay.
'past' becomes 'old-time' (I've seen this in another clue).
'his' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'oldtime'+'r'='OLD-TIMER'
Can you help me to learn more?
(Other definitions for old timer that I've seen before include "clock?" , "Grandad" , "One's no longer in service" , "Hourglass" , "person of experience" .)