Waltz's intro with cool beat (4)
I believe the answer is:
whip
'beat' is the definition.
(whipping is a kind of beating)
'waltz's intro with cool' is the wordplay.
'intro' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'cool' becomes 'hip' (hip can mean cool or fashionable).
The first letter of 'waltzs' is 'w'.
'w'+'hip'='WHIP'
(Other definitions for whip that I've seen before include "MP enforcing discipline" , "He maintains discipline among members" , "Legislator appointed by a party to enforce discipline" , "The one who enforces discipline in a political party" , "Strike with lash or rod" .)