A little boy, Winchester’s sixth former (8)
I believe the answer is:
sometime
'former' is the definition.
(I know that former can be written as sometime)
'a little boy winchester's sixth' is the wordplay.
'a little' becomes 'some' (I've seen this in another clue).
'boy' becomes 'tim' (short for Timothy).
'winchester's sixth' becomes 'e' (6th letter of 'winchester').
'some'+'tim'+'e'='SOMETIME'
(Other definitions for sometime that I've seen before include "At a future unknown period" , "One day" , "ex?" , "Previous" , "An indeterminate date in the future" .)