Speechless about a temperature change (6)
I believe the answer is:
mutate
'change' is the definition.
(mutating is a kind of changing)
'speechless about a temperature' is the wordplay.
'speechless' becomes 'mute' (I've seen this before).
'about' indicates putting letters inside.
'temperature' becomes 't' (abbreviation).
'a'+'t'='at'
'mute' enclosing 'at' is 'MUTATE'.
(Other definitions for mutate that I've seen before include "Alter genetically" , "Undergo alteration in form" , "Undergo genetic change" , "Undergo cellular change" , "Me taut? (anag) - change fundamentally" .)