Hiss! At the end of summer, a scorcher (7)
I believe the answer is:
sizzler
'a scorcher' is the definition.
(thesaurus)
'hiss at the end of summer' is the wordplay.
'hiss' becomes 'sizzle' (I have seen 'Hot hiss ' mean 'sizzle' so perhaps 'hiss' could also mean 'sizzle').
'at the end of' suggests the final letters.
The final letter of 'summer' is 'r'.
'sizzle'+'r'='SIZZLER'
(Other definitions for sizzler that I've seen before include "Exceptionally hot day" , "Striking success - very hot day" .)