Decline to change a beat (5)
I believe the answer is:
abate
'decline' is the definition.
(I've seen this before)
'change a beat' is the wordplay.
'change' indicates an anagram.
'a'+'beat'='abeat'
'abeat' is an anagram of 'ABATE'.
'to' acts as a link.
(Other definitions for abate that I've seen before include "Demolish" , "Let up or die away" , "Subside, grow less" , "Lessen - cease" , "Become less strong like storm" .)