Crossword Genius

Travelling partition moves when it isn't there (2,1,4)

Ross

I believe the answer is:

on a trip

'partition moves when it isn't there' is the definition.
I can't tell whether this defines the answer.

'travelling' is the wordplay.
I cannot quite see how this works, but
'travelling' could be 'trip' (trip is a kind of travelling) and 'trip' is found in the answer.
This may be the basis of clue (or it may be nonsense).

I've seen this clue in The Independent.
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