Theatre objected to supporting extra appearing more often than expected (15)

'more often than expected' is the definition.
I can't tell whether this definition defines the answer.

'theatre objected to supporting extra' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'objected to' becomes 'resented' (I can't explain this - if you can you should give a lot more credence to this answer).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'extra' becomes 'over' (both can mean left over).
'rep'+'resented'='represented'
'represented' put after 'over' is 'OVERREPRESENTED'.

'appearing' is the link.

Can you help me to learn more?