Unpredictable blunder by a jerk (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
(synonyms)
'blunder by a jerk' is the wordplay.
'blunder' becomes 'err' (to err is to make a mistake or blunder).
'by' means one lot of letters go next to another.
'jerk' becomes 'tic' (jerking movement by the body).
'err'+'a'+'tic'='ERRATIC'
(Other definitions for erratic that I've seen before include "Volatile" , "Irregular in motion or habit" , "Variable" , "Inconsistent, shifting" , "Irregular, capricious" .)