The most common and most likely answer to this crossword clue is the 7 letter word OESTRUS
I've seen this crossword clue in Financial Times
O
E
S
T
R
U
S
More clues leading to the result OESTRUS
Buddy explains!

The straight part
We know that "heat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case heat = oestrus.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 OESTRUS mean?
OESTRUS noun- A biting fly of the genus Oestrus; a botfly.
- A bite or sting.
- A passion or frenzy.
- A female animal's readiness to mate; heat, rut.