"On top of which ... " (4)
I believe the answer is:
also
'on top of which' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for also that I've seen before include "Additionally" , "'Besides, moreover (4)'" , "into the bargain" , "an indication of more to come" , "item's rarely used now for this" .)