Deadly heat waves in the outskirts of Liverpool (6)
I believe the answer is:
lethal
'deadly' is the definition.
('lethal' can be a synonym of 'deadly')
'heat waves in the outskirts of liverpool' is the wordplay.
'waves' indicates anagramming the letters.
'in the' means one lot of letters goes inside another.
'outskirts of liverpool' becomes 'll' (first and last letters of 'Liverpool').
'heat' with letters rearranged gives 'etha'.
'etha' placed inside 'll' is 'LETHAL'.
(Other definitions for lethal that I've seen before include "Liable to cause death" , "Destructive" , "All the change can be deadly." , "Able to cause death" , "Capable of causing death" .)