Pumps gas into defined region on return (7)
I believe the answer is:
aerates
'pumps gas' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how one could define the other.
'defined region on return' is the wordplay.
'defined' becomes 'set' (I am not sure about this - if you are sure you should believe this answer much more).
'region' becomes 'area' (area is a kind of region).
'on return' says the letters should be written in reverse.
'set'+'area'='setarea'
'setarea' back-to-front is 'AERATES'.
'into' is the link.
Can you help me to learn more?
(Other definitions for aerates that I've seen before include "Puts fizz into a drink" , "fans" , "Brings gas" , "uses fork to do it?" , "Pumps carbon dioxide into" .)