Cook a bit of grub badly (5)
I believe the answer is:
grill
'cook' is the definition.
(grilling is a form of cooking)
'a bit of grub badly' is the wordplay.
'a bit of grub' becomes 'gr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'badly' becomes 'i'll' ('ill' can be adverbial as in 'ill-advised').
'gr'+'ill'='GRILL'
Can you help me to learn more?
(Other definitions for grill that I've seen before include "Screen" , "interrogation?" , "Broil" , "Interrogate - cook" , "rack" .)