Formerly on a future occasion! (8)
I believe the answer is:
sometime
'formerly' is the definition.
(I've seen this before)
'on a future occasion' is the wordplay.
'on' is a charade indicator (letters next to each other).
'a future' becomes 'some' (I am not sure about this - if you are sure you should believe this answer much more).
'occasion' becomes 'time' (I've seen this in another clue).
'some' put next to 'time' is 'SOMETIME'.
Can you help me to learn more?
(Other definitions for sometime that I've seen before include "eventually" , "Previous" , "In the future, we don't know when" , "Former; occasional (US)" , "ex?" .)