Strong fastening (9)
I believe the answer is:
strapping
I believe this clue is a double definition.
'strong' is the first definition.
('strapping' can be a synonym of 'strong')
'fastening' is the second definition.
(strapping is a kind of fastening)
(Other definitions for strapping that I've seen before include "stalwart" , "Brawny" , "Big" , "Burly" , "Tying up" .)