Add too much to sketch and get a negative balance (8)
I believe the answer is:
overdraw
I believe this clue is a double definition.
'add too much to sketch' is the first definition.
(draw too much)
'get a negative balance' is the second definition.
(go into a bank overdraft)
'and' is the link.
(Other definitions for overdraw that I've seen before include "Use more money than you have in the bank" , "Go into the red" , "Exaggerate" , "Stretch the truth" , "Get into the red" .)