Snowball
I believe the answer is:
'snowball' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for escalate that I've seen before include "Cause to increase, say dispute (8)." , "Increase in intensity or extent, like trouble brewing" , "Tease Cal badly and mount up" , "Rapidly increase in intensity" , "mushroom" .)