A post about Celsius scale (6)
I believe the answer is:
ascend
'scale' is the definition.
(as in scaling a mountain)
'a post about celsius' is the wordplay.
'post' becomes 'send' (synonyms).
'about' is an insertion indicator.
'celsius' becomes 'c' (abbreviation for Celsius).
'send' going around 'c' is 'scend'.
'a'+'scend'='ASCEND'
(Other definitions for ascend that I've seen before include "Scale" , "Soar" , "rise in status" , "shift to a higher plane" , "Come up" .)