Virtually immaculate conception (4)
I believe the answer is:
idea
'conception' is the definition.
(I've seen this in another clue)
'virtually immaculate' is the wordplay.
'virtually' means to remove the last letter (virtually all of the word, but not the last letter).
'immaculate' becomes 'ideal' (I've seen this before).
'ideal' with its final letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "intellectual product" , "Notion, concept" , "Suggestion" , "Creative spark" , "a wheeze" .)