Great deal of flotsam (3)
I believe the answer is:
lot
'great deal' is the definition.
(both can mean a large amount)
'of flotsam' is the wordplay.
'of' indicates a hidden word.
'LOT' is hidden within 'flotsam'.
(Other definitions for lot that I've seen before include "Great amount" , "man rescued by angels" , "one whose wife should be remembered" , "Genesis character" , "Could there be many at the auction?" .)