Extra exhaust put in and modified (3-2)
I believe the answer is:
add-on
'extra' is the definition.
(I've seen this before)
'exhaust put in and modified' is the wordplay.
'exhaust' becomes 'do'.
'put in' indicates putting letters inside.
'modified' indicates anagramming the letters.
'and' is an anagram of 'adn'.
'do' put into 'adn' is 'ADD-ON'.
(Other definitions for add-on that I've seen before include "Extra bit" , "Feature supplementing standard model" , "Accessory device" , "Extra unit, peripheral" , "Feature bolted on to a standard model or package" .)