"This is important" (4,4)
I believe the answer is:
take note
'this is important' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for take note that I've seen before include "what the stenographer does" , "Observe and absorb - jot it down" , "mark?" , "Pay attention" , "Observe closely" .)