It injects air into something flat? (4)
I believe the answer is:
pump
This clue could be a double definition.
'it injects air into something' is the first definition.
The definition suggests a singular noun which matches the answer.
'flat?' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for pump that I've seen before include "Compressing machine" , "Question - means of inflation" , "Fluid mover" , "Fluid moving device" , "Device for moving fluids" .)