Abrupt decline, possibly becoming erratic (13)
I believe the answer is:
unpredictable
'erratic' is the definition.
('unpredictable' can be a synonym of 'erratic')
'abrupt decline possibly' is the wordplay.
'possibly' is an anagram indicator.
'abrupt'+'decline'='abruptdecline'
'abruptdecline' with letters rearranged gives 'UNPREDICTABLE'.
'becoming' acts as a link.
(Other definitions for unpredictable that I've seen before include "Capricious" , "erratic" , "Changeable" .)