Abrupt decline strangely random (13)
I believe the answer is:
unpredictable
'random' is the definition.
(I know that random can be written as unpredictable)
'abrupt decline strangely' is the wordplay.
'strangely' is an anagram indicator.
'abrupt'+'decline'='abruptdecline'
'abruptdecline' anagrammed gives 'UNPREDICTABLE'.
(Other definitions for unpredictable that I've seen before include "Changeable" , "Capricious" , "erratic" .)