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