Clown leaving rubbish under jetty (7)
I believe the answer is:
pierrot
'clown' is the definition.
(I've seen this before)
'rubbish under jetty' is the wordplay.
'rubbish' becomes 'rot' (I've seen this before).
'under' means one lot of letters go next to another.
'jetty' becomes 'pier' (I've seen this before).
'rot' after 'pier' is 'PIERROT'.
'leaving' acts as a link.
(Other definitions for pierrot that I've seen before include "Seaside clown" , "he's white-faced at work" , "Clown from French pantomime with white face and costume - tripe or what?" , "Sad white-faced clown" , "Period; incantation" .)