Saying nothing about a temperature change (6)
I believe the answer is:
mutate
'change' is the definition.
(mutating is a kind of changing)
'saying nothing about a temperature' is the wordplay.
'saying nothing' becomes 'mute' (I've seen this in other clues).
'about' is an insertion indicator.
'temperature' becomes 't' (abbreviation).
'a'+'t'='at'
'mute' placed around 'at' is 'MUTATE'.
(Other definitions for mutate that I've seen before include "Change into something different" , "Change in genetic way, perhaps from radiation" , "Convert" , "Change into a variant form" , "Change in basic nature" .)