Headbutt PM in dark (3-5)

I believe the answer is:

nut-brown

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'dark' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'headbutt pm' is the wordplay.
'headbutt' becomes 'nut' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pm' becomes 'brown' (I have seen 'Former PM ' mean 'brown' so perhaps 'pm' could also mean 'brown').
'nut'+'brown'='NUT-BROWN'

'in' acts as a link.

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I've seen this clue in The Guardian.

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