Fastener on top, new dark colour (3-5)

I believe the answer is:

nut-brown

'dark colour' is the definition.
(I've seen this before)

'fastener on top new' is the wordplay.
'fastener' becomes 'nut' (a nut fastens things).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'top' becomes 'brow' (brow can mean a summit or peak).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'nut'+'brow'+'n'='NUT-BROWN'

I've seen this clue in The Guardian.

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