Useful in case of flat or round belly (5,4)
I believe the answer is:
spare tyre
I believe this clue is a double definition.
'useful in case of flat' is the first definition.
(a spare tyre is useful if you get a flat tyre)
'round belly' is the second definition.
(spare tyre can mean belly fat)
'or' acts as a link.
(Other definitions for spare tyre that I've seen before include "Gastric band" , "The fifth wheel?" , "Form of extra weight" , "Love handles" , "large corporation" .)