Replace the air and do a U-turn (6,4,4)
I believe the answer is:
change ones tune
'replace the air and do a u-turn' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'replace the air and do a u-turn' is the wordplay.
'replace' becomes 'change' (changing is a kind of replacing).
'the air' becomes 'tune' (air can mean a piece of music).
'and' means one lot of letters go next to another.
'do a u-turn' becomes 'ones' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tune' put after 'ones' is 'onestune'.
'change'+'onestune'='CHANGE ONES TUNE'
Can you help me to learn more?
(Other definitions for change ones tune that I've seen before include "to get fresh air?" , "Cross the floor perhaps" , "put new record on?" , "Abandon current position" , "Alter attitude" .)