Unconsummated threesome breaks rules (8)

I believe the answer is:

theorems

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

'rules' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for theorems that I've seen before include "Demonstrable principles" , "Propositions proved by chain of reasoning" , "Propositions proved by chains of reasoning" , "Mathematical propositions" , "Rules" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app