Over river, remedied a poorly run upcoming music venue (5,5,5)
I believe the answer is:
royal opera house
'over river' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'remedied a poorly run upcoming music venue' is the wordplay.
'remedied' is an anagram indicator.
'run' becomes 'hare' (hare can mean to run very fast).
'upcoming' says the letters should be written in reverse (in a down clue coming up is backwards).
'music venue' becomes 'ouse' (I can't explain this - if you can you should give a lot more credence to this answer).
'hare' back-to-front is 'erah'.
'a'+'poorly'='apoorly'
'apoorly' with letters rearranged gives 'royalop'.
'royalop'+'erah'+'ouse'='ROYAL OPERA HOUSE'
Can you help me to learn more?
(Another definition for royal opera house that I've seen is " Covent Garden - with 23d,24d,26d".)