It's subject to inflation (4)
I believe the answer is:
tyre
'it's subject to inflation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tyre that I've seen before include "Wheel's rubber tube" , "Seaport once famed for its purple dye" , "Rubber surround on a wheel" , "Something with a tread" , "Rubber wheel-ring" .)