A blow in the tub if far too hot! (4)

I believe the answer is:

biff

I'm a little stuck... Click here to teach me more about this clue!

'a blow' is the definition.
(I know that biff is a type of blow)

'the tub if far too hot' is the wordplay.
'BIFF' is hidden in amongst the letters.
But, I'm uncertain how this is indicated.

'in' is the link.

Can you help me to learn more?

(Other definitions for biff that I've seen before include "Hard blow with fist" , "Punch" , "strike" .)

I've seen this clue in the Sydney Morning Herald.

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