Celebrity snubbed over many items in the main (7)
I believe the answer is:
'items in the main' is the definition.
(items floating in the sea - main can mean the sea)
'celebrity snubbed over many' is the wordplay.
'celebrity' becomes 'fame' (celebrity can mean the quality of being famous).
'snubbed' means to remove the last letter (could historically mean cutting the end off).
'over' is an insertion indicator.
'many' becomes 'lots' (lots of something).
'fame' with its last letter taken off is 'fam'.
'fam' placed around 'lots' is 'FLOTSAM'.
(Other definitions for flotsam that I've seen before include "Wreck" , "Rubbish on sea" , "Wreckage adrift at sea" , "Things found floating from a ship" , "Vagrants" .)