Pay for holiday, you say? That'll slow you down (4,5)

I believe the answer is:

foot brake

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'that'll slow you down' is the definition.
The definition suggests a singular noun which matches the answer.

'pay for holiday you say?' is the wordplay.
'pay for' becomes 'foot' (as in to foot the bill).
'holiday' becomes 'break' (I've seen this before).
'you say?' shows a homophone (sound like).
'break' is a homophone of 'brake'.
'foot'+'brake'='FOOT BRAKE'

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I've seen this clue in The Independent.

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