Creature possibly dead by summer's end (5)
I believe the answer is:
adder
'creature' is the definition.
(I have seen 'venomous creature' mean 'adder' so perhaps 'creature' could also mean 'adder')
'possibly dead by summer's end' is the wordplay.
'possibly' indicates an anagram.
'by' means one lot of letters go next to another.
'end' indicates one should take the final letters.
The last letter of 'summer' is 'r'.
'dead' is an anagram of 'adde'.
'adde'+'r'='ADDER'
(Other definitions for adder that I've seen before include "An arithmetic snake" , "Britain's only venomous snake" , "One contributing more" , "venomous creature" , "Venomous sort" .)