It fixes something round one's neck (6)
I believe the answer is:
tiepin
This clue could be a double definition.
'it fixes something' is the first definition.
The definition suggests a singular noun which matches the answer.
'one's neck' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'round' acts as a link.
Can you help me to learn more?
(Other definitions for tiepin that I've seen before include "security at front" , "clothing accessory" , "Gent's accessory for an accessory" , "Fixer for neckwear" , "Neckwear securer" .)