In hairdressing, the best union (7)
I believe the answer is:
topknot
'in hairdressing' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'best union' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'union' becomes 'knot' (I can't justify this - if you can you should give a lot more credence to this answer).
'top'+'knot'='TOPKNOT'
'the' is the link.
Can you help me to learn more?
(Other definitions for topknot that I've seen before include "Hair extension" , "Decorative bow on head" , "Crest of hair or feathers on the crown of the head" , "Crest - tuft of hair" , "Tuft or crest on crown of head" .)