A great deal of flotsam (3)
I believe the answer is:
lot
'a great deal' is the definition.
(both can mean a large amount)
'of flotsam' is the wordplay.
'of' indicates the answer is hidden within the clue.
'LOT' is hidden in the letters of 'flotsam'.
(Other definitions for lot that I've seen before include "Abraham's nephew - assigned share" , "Share - tons" , "Sale item(s); destiny" , "Destiny - auction item(s)" , "Survivor in Sodom" .)