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' says the answer is hidden in the clue.
'LOT' can be found hidden inside '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" .)