Wrong about temperature, which won't be perishing (8)
I believe the answer is:
immortal
'which won't be perishing' is the definition.
'wrong about temperature' is the wordplay.
'wrong' becomes 'immoral' (similar in meaning).
'about' means one lot of letters goes inside another.
'temperature' becomes 't' (abbreviation).
'immoral' placed around 't' is 'IMMORTAL'.
(Other definitions for immortal that I've seen before include "Imperishable" , "Living for ever" , "never to be forgotten" , "God" , "Never dying" .)