Get more intense (6)

I think it could be either:

Which length description are you looking for?:

4,2

heat up

'get more intense' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for heat up that I've seen before include "Make more intense" , "Warm through" .)

I've seen this clue in The Wall Street Journal.

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