Angry after competitor's opening broke the rules (7)
I believe the answer is:
cheated
'broke the rules' is the definition.
(to cheat is to break the rules of a game)
'angry after competitor's opening' is the wordplay.
'angry' becomes 'heated' (I've seen this before).
'after' means one lot of letters go next to another.
'competitor's opening' becomes 'c' (I can't justify this - if you can you should give a lot more credence to this answer).
'heated' after 'c' is 'CHEATED'.
Can you help me to learn more?
(Other definitions for cheated that I've seen before include "taken for a ride" , "Defeated through deceit" , "Behaved dishonestly" , "stung" .)