The twit with the companion, at the piano (5)
I believe the answer is:
chump
'the twit' is the definition.
(thesaurus)
'with the companion at the piano' is the wordplay.
'with' is a charade indicator (letters next to each other).
'the companion at' becomes 'chum' (I can't explain this - if you can you should believe this answer much more).
'the piano' becomes 'p' (musical abbreviation).
'chum'+'p' is 'CHUMP'.
Can you help me to learn more?
(Other definitions for chump that I've seen before include "Foolish person" , "Blockhead" , "Mug, patsy, sucker" , "Fool - sort of chop" , "Dolt" .)