Bad driver had changed into jumper? Good! (4,3)
I believe the answer is:
road hog
'bad driver' is the definition.
(I've seen this before)
'had changed into jumper? good' is the wordplay.
'changed' is an anagram indicator.
'into' means one lot of letters goes inside another.
'jumper?' becomes 'roo' (short for kangaroo, something which jumps).
'good' becomes 'g' (abbreviation).
'had' anagrammed gives 'adh'.
'adh' put within 'roo' is 'roadho'.
'roadho'+'g'='ROAD-HOG'
(Other definitions for road hog that I've seen before include "Inconsiderate motorist" , "A careless and selfish driver, the swine" , "Discourteous driver" , "Selfishly aggressive driver" , "Bad driver" .)