Plus I'm travelling in one without having planned to (2,7)

I believe the answer is:

on impulse

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'without having planned to' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'plus i'm travelling in one' is the wordplay.
'travelling' indicates an anagram.
'in' indicates putting letters inside.
'plus'+'im'='plusim'
'plusim' is an anagram of 'impuls'.
'impuls' inserted within 'one' is 'ON IMPULSE'.

Can you help me to learn more?

(Other definitions for on impulse that I've seen before include "Spontaneously" , "Without thinking" , "without forethought" , "Without premeditation or planning, perhaps buying" .)

I've seen this clue in the Evening Standard.

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