The loud beat changed quickly (2,3,6)
I believe the answer is:
at the double
'quickly' is the definition.
(I've seen this before)
'the loud beat changed' is the wordplay.
'changed' indicates an anagram.
'the'+'loud'+'beat'='theloudbeat'
'theloudbeat' with letters rearranged gives 'AT THE DOUBLE'.
(Other definitions for at the double that I've seen before include "travelling very fast" , "Able (to do something)" , "in half the usual time?" , "dart ultimately aimed here?" , "Where arrow is aimed, initially" .)