Eggs on some cheese -- grub's up! (5)
I believe the answer is:
urges
'eggs on' is the definition.
(I've seen this before)
'some cheese grub's up' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'segru' is hidden within 'cheese grubs'.
'segru' backwards is 'URGES'.
(Other definitions for urges that I've seen before include "Eggs on" , "Drives" , "Encourages" , "Strong impulses" , "Exhorts or prods" .)