Having intelligence and success - that's cool (4,2)
I believe the answer is:
with it
'cool' is the definition.
(I've seen this in another clue)
'having intelligence and success' is the wordplay.
'having intelligence' becomes 'wit' (I can't explain this - if you can you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'success' becomes 'hit' (a hit is a commercial success).
'wit'+'hit'='WITH-IT'
'that's' is the link.
Can you help me to learn more?
(Other definitions for with it that I've seen before include "Up to date" , "Fashion conscious; alert and aware" , "as well?" , "Fashionable" , "Intelligently aware; trendy" .)