Understand increase (10)
I believe the answer is:
appreciate
I believe this is a double definition.
'understand' is the first definition.
(appreciating is a kind of understanding)
'increase' is the second definition.
(I know that appreciate is a more specific form of the action increase)
(Other definitions for appreciate that I've seen before include "Welcome" , "increase in price" , "(Of asset) rise in value" , "Take full account of" , "Enjoy" .)