Bad film of oil obliterating lake (4)
I believe the answer is:
sick
'bad' is the definition.
(similar in meaning)
'film of oil obliterating lake' is the wordplay.
'film of oil' becomes 'slick' (as in an oil slick).
'obliterating' is a deletion indicator.
'lake' becomes 'l' (geographical abbreviation).
'slick' with 'l' taken away is 'SICK'.
(Other definitions for sick that I've seen before include "Poorly" , "Ill, unwell" , "Ill; perverted" , "well? Hardly" , "Ailing" .)