Contribute 100? Doubly cool! (4,2)
I believe the answer is:
chip in
'contribute' is the definition.
(I know that contribute can be written as chip in)
'100? doubly cool' is the wordplay.
'c00? doubly' becomes 'chip' (I am not sure about this - if you are sure you should believe this answer much more).
'cool' becomes 'in' ('in' can mean fashionable or cool).
'chip'+'in'='CHIP-IN'
Can you help me to learn more?
(Other definitions for chip in that I've seen before include "Contribute a piece of potato?" , "Interpose" , "Pay part of the cost of something" , "hole the ball" , "add to the kitty" .)