Be highly observant (4,2)
I believe the answer is:
look up
'be highly observant' is the definition.
I can't tell whether this defines the answer.
'be highly observant' is the wordplay.
'be' becomes 'look' (looking is a kind of being).
'highly observant' becomes 'up' (I can't explain this - if you can you should believe this answer much more).
'look'+'up'='LOOK UP'
Can you help me to learn more?
(Other definitions for look up that I've seen before include "Gaze aloft or go to visit" , "Search for as in dictionary (4,2)" , "Search in dictionary or go to visit" , "Search for information as in encyclopedia" , "Raise eyes; check (reference)" .)