Cleaner exhaust with nothing inside (3)
I believe the answer is:
oap
'cleaner exhaust with nothing inside' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'cleaner exhaust with nothing inside' is the wordplay.
'cleaner exhaust' becomes 'op' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' indicates putting letters inside.
'nothing' becomes 'scratch' (as in making something 'from scratch').
'inside' means to look at the middle letters.
The middle letter of 'scratch' is 'a'.
'op' enclosing 'a' is 'OAP'.
Can you help me to learn more?
(Another definition for oap that I've seen is " senior citizen".)