Certainly on one's own this time (5)
I believe the answer is:
today
'this time' is the definition.
(today is a kind of time)
'certainly on one's own' is the wordplay.
'certainly' becomes 'ay' (variant spelling of 'aye').
'on' means one lot of letters go next to another.
'one's own' becomes 'tod' (I can't explain this - if you can you should believe this answer much more).
'ay' after 'tod' is 'TODAY'.
Can you help me to learn more?
(Other definitions for today that I've seen before include "Nowadays" , "At the present time, in general" , "Presently" , "one of our papers, once" , "Modern times" .)