Annoyed when obliged to accept a rule (5)
I believe the answer is:
tried
'annoyed when obliged to accept a rule' is the definition.
(I've seen this before)
'to accept a rule' is the wordplay.
'to' becomes 'tied' (I can't justify this - if you can you should give a lot more credence to this answer).
'accept' means one lot of letters goes inside another.
'a rule' becomes 'r' (legal abbreviation).
'tied' placed around 'r' is 'TRIED'.
Can you help me to learn more?
(Other definitions for tried that I've seen before include "Annoyed when obliged to accept a rule" , "checked out" , "Put to the test" , "Made an effort" , "Had a crack" .)