Leave egghead entirely (5)
I believe the answer is:
quite
'entirely' is the definition.
(I've seen this in another clue)
'leave egghead' is the wordplay.
'leave' becomes 'quit' (quitting is a kind of leaving).
'egghead' becomes 'e' (head letter of 'egg').
'quit'+'e'='QUITE'
(Other definitions for quite that I've seen before include "Rather" , "Fairly; indeed" , "Somewhat; completely" , "Completely -- to some extent" , "To a degree" .)