Crossword Genius

On heat, bother male bears (7)

Ross

I believe the answer is:

thermal

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

'on heat' is the definition.
(I know this)

'bother male bears' is the wordplay.
'THERMAL' is hidden within the letters.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for thermal that I've seen before include "Her malt became heat-related" , "Upward current of warm air" , "which can take a glider up" , "Warm or hot - underwear?" , "Related to heat" .)

I've seen this clue in The Independent.
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