Piano one in riotous club known to all (6)
I believe the answer is:
public
'known to all' is the definition.
'piano one in riotous club' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'riotous' is an anagram indicator.
'club' anagrammed gives 'ublc'.
'i' placed into 'ublc' is 'ublic'.
'p'+'ublic'='PUBLIC'
(Other definitions for public that I've seen before include "Overt" , "General - population" , "Strangely enough, these English schools are not" , "In open view" , "Relating to the general community" .)