Declines to upset at base (6)
I believe the answer is:
abates
'declines' is the definition.
(I've seen this in another clue)
'upset at base' is the wordplay.
'upset' indicates an anagram.
'at'+'base'='atbase'
'atbase' with letters rearranged gives 'ABATES'.
'to' is the link.
(Other definitions for abates that I've seen before include "Goes down like storm" , "Becomes less severe or strong" , "Diminishes" , "Slacks off" , "moderates" .)