Part of a pneumatic tyre (5,4)
I believe the answer is:
inner tube
'part of a pneumatic tyre' is the definition.
('inner tube' is part of 'pneumatic tyre')
This is all the clue.
(Other definitions for inner tube that I've seen before include "inflatable ring" , "One used to inflation going by bicycle" , "Circle line?" , "rubber ring" , "What cyclist pumps up" .)