Variable, strangely abrupt decline (13)
I believe the answer is:
unpredictable
'variable' is the definition.
(thesaurus)
'strangely abrupt decline' is the wordplay.
'strangely' indicates an anagram.
'abrupt'+'decline'='abruptdecline'
'abruptdecline' with letters rearranged gives 'UNPREDICTABLE'.
(Other definitions for unpredictable that I've seen before include "erratic" , "Changeable" , "Capricious" .)