In dark, wounded badly, duck remains flattened (8)
I believe the answer is:
roadkill
'remains flattened' is the definition.
(remains of an animal)
'in dark wounded badly duck' is the wordplay.
'in' means one lot of letters goes inside another.
'wounded' is an anagram indicator.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'dark' anagrammed gives '[dark]'.
'[dark]'+'ill'='radkill'
'radkill' enclosing 'o' is 'ROADKILL'.
(Other definitions for roadkill that I've seen before include "Animals slain by vehicles" , "A passing motoring phenomenon" , "Animal slain by cars" , "Creatures squashed on the motorway?" .)