Wreckage of mast drifting across lake (7)
I believe the answer is:
flotsam
'wreckage' is the definition.
(flotsam is a kind of wreckage)
'of mast drifting across lake' is the wordplay.
'drifting' indicates anagramming the letters.
'across' is an insertion indicator.
'lake' becomes 'l'.
'of'+'mast'='ofmast'
'ofmast' is an anagram of 'fotsam'.
'fotsam' going around 'l' is 'FLOTSAM'.
(Other definitions for flotsam that I've seen before include "The floating wreckage of a ship" , "Rubbish on sea" , "Things found floating from a ship" , "Wreck" , "Detritus" .)