The wreckage floats, drifting a mile on (7)
I believe the answer is:
flotsam
'the wreckage' is the definition.
(flotsam is a kind of wreckage)
'floats drifting a mile on' is the wordplay.
'drifting' indicates anagramming the letters.
'a mile' becomes 'm' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'floats' with letters rearranged gives 'flotsa'.
'flotsa'+'m'='FLOTSAM'
(Other definitions for flotsam that I've seen before include "Goods lost" , "Things found floating from a ship" , "Wreckage adrift at sea" , "Detritus" , "Wreck" .)