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How about this for devious:

ERNIE is a computer, it uses maths, "-" is the sign for subtraction in maths, so...

7D 19 - in 13

"OMEN" is the definition and "-" is a "SIGN" used by ERNIE the computer.

Very happy with that! Good thinking and certainly devious. Thank you, seamus, ayrshire

Thank you, Seamus - successfully done! As an ancient grandma (well, elderly!) I'm happy to learn new tricks!

I have the only answer which fits for 18D but I'm still puzzled by "function cycling" in the clue. Any hints?

The answer is an anagram of a mathematical function ('cycling' indicating an anagram).

Thanks, I see it now. A long time since I did A-level trig!

Seamus
This is no worse than my own suggestion but ...
strctly speaking ERNIE is not a computer but rather a random number generator and I don't think the minus sign (-) therfore has much to do with its operation. It doesn't 'do' maths.
The connection between 13A and 7D defeats me.

It was a very tongue-in-cheek suggestion. Sadly there's no smileys on this forum, so I'll have to make do with the old Usenet ones ;-)