I tend to say that an animal is "in" heat instead of "on". That's the French language ("en" chaleur) interefering with my English. Hence my initial confusion.
Quite a few online dictionaries have "on (in) heat", or "on heat or in heat" as an entry. I feel a little less lonely no ha ha
Thank you once again for your help.
Thank you. I started with easy cryptics about 11 or 12 years ago and upgraded to the Guardian's 10 years ago. So I've got quite a bit of experience to help me now.