... a rearing appearance, an improvement (6)
I believe the answer is:
repair
'an improvement' is the definition.
(I know that repair is a type of improvement)
'a rearing appearance' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'rearing' shows that the letters should be reversed in order (as in a horse rearing up).
'appearance' becomes 'air' (both can mean an impression or character).
'per' backwards is 'rep'.
'rep'+'air'='REPAIR'
(Other definitions for repair that I've seen before include "Restore to working order" , "Mend, put right" , "service" , "It may be effected by" , "Mend - go" .)