The most common and most likely answer to this crossword clue is the 12 letter word THERMOCOUPLE
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "temperature-sensing device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case temperature-sensing device = thermocouple.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THERMOCOUPLE mean?
THERMOCOUPLE noun- A transducer consisting of two different metals welded together at each end; a voltage is produced that is proportional to the difference in temperature between the two junctions (one of which is normally held at a known temperature)