Fastener proves to be snag (5)
I believe the answer is:
hitch
'to be snag' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for hitch that I've seen before include "Marry" , "Technical problem" , "Tether (rope); difficulty" , "Snag; tie up (animal)" , "Thumb (a lift)" .)