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' is an anagram indicator.
'a mile' becomes 'm' (common abbreviation).
'on' means one lot of letters go next to another.
'floats' is an anagram of '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" .)