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' says the answer is hidden in the clue.
'LOT' is hidden in the letters of '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?" .)