Intensify when there’s an invitation to board (4,2)

I believe the answer is:

step up

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'intensify' is the definition.
(I know that intensify can be written as step up)

I can't explain the remainder of the clue.

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(Other definitions for step up that I've seen before include "Advancement (on a ladder or career)" , "Sudden increase (in rate, output, etc)" , "To be promoted" , "Boost" , "- Bezukhov (War and Peace)" .)

I've seen this clue in the Evening Standard.

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