Change down into this mean outfit (3,4)
I believe the answer is:
low gear
'change down into this' is the definition.
(change gear while driving)
'mean outfit' is the wordplay.
'mean' becomes 'low' (both can mean nasty or dishonourable).
'outfit' becomes 'gear' (I've seen this in another clue).
'low'+'gear'='LOW GEAR'
(Other definitions for low gear that I've seen before include "which is needed for hills" , "in which one may crawl" , "one that suits one crawling along" , "Shoes and socks" , "Car setting for hills" .)