A good bit of cash to burn! (3)
I believe the answer is:
gas
'burn' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how they can define each other.
'a good bit of cash' is the wordplay.
'a good' becomes 'g' (abbreviation).
'bit of cash' becomes 'as' (I can't justify this - if you can you should believe this answer much more).
'g'+'as'='GAS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "Eg, nitrogen" , "Possibly noble" , "Chat (colloq.)" , "It may be natural" , "Something exciting" .)