Empty mosquito repellent aerosol could be lethal (6)
I believe the answer is:
mortal
'be lethal' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for mortal that I've seen before include "liable to die" , "Destined to die" , "Certain to die" , "Very drunk < person" , "Subject to death, like all mankind" .)