Egg on hamburger? Not entirely! (4)
I believe the answer is:
urge
'egg' is the definition.
(I've seen this in another clue)
'on hamburger? not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'URGE' can be found hidden inside 'on hamburger'.
(Other definitions for urge that I've seen before include "Encourage (to do something)" , "'Exhort, press (4)'" , "Try hard to persuade, advise earnestly" , "Propel, drive onward" , "Capital of Colombia" .)