Hot lines upset Rose Smith (9)

I believe the answer is:

isotherms

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

'hot lines' is the definition.
'isotherms' can be an answer for 'lines' (isotherm is a kind of line). I am unsure of the 'hot' bit.

'upset rose smith' is the wordplay.
'upset' is an anagram indicator.
'rose'+'smith'='rosesmith'
'rosesmith' with letters rearranged gives 'ISOTHERMS'.

Can you help me to learn more?

(Another definition for isotherms that I've seen is " lines on maps".)

I've seen this clue in The Globe and Mail.

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