Somebody impregnated by leader of Conservatives in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for once that I've seen before include "yesterday" , "in days of yore?" , "Some time back" , "Formerly - not repeated" , "... upon a time for the storyteller" .)