Rubber or plastic ring lining a hole (7)
I believe the answer is:
grommet
(Other definitions for grommet that I've seen before include "Ring of rubber to line a hole and prevent chafing" , "Ring for hole - tube for eardrum" , "Tube in eardrum to drain and ventilate ear" , "Ring shaped to line a small hole" , "Washer; eyelet" .)