Old seaside entertainer's nonsense on the pier (7)
I believe the answer is:
pierrot
'old seaside entertainer's' is the definition.
'pierrot' can be an answer for 'entertainer's' (stock theatrical character). I'm not sure about the 'old seaside' bit.
'nonsense on the pier' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'on' is a charade indicator (letters next to each other).
'rot' after 'pier' is 'PIERROT'.
Can you help me to learn more?
(Other definitions for pierrot that I've seen before include "Aged entertainer" , "Clown from French panto - tripe or what" , "Period; incantation" , "Male clown from French pantomime" , "French stage character with white face and costume" .)