This thing is boiling, you understand (3)
I believe the answer is:
see
'understand' is the definition.
(I know that see is a kind of the understand)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for see that I've seen before include "Go out with; notice" , "make out" , "around cathedral" , "Observe - diocese" , "Bishops office" .)