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' says to put letters next to each other.
'competitor's opening' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'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" .)