Physicist's scheme united Celsius and Kelvin (6)
I believe the answer is:
planck
'physicist's' is the definition.
(I've seen this before)
'scheme united celsius and kelvin' is the wordplay.
'scheme' becomes 'plan' (synonyms).
'united' says to put letters next to each other.
'celsius' becomes 'c' (abbreviation for Celsius).
'and' is a charade indicator (letters next to each other).
'kelvin' becomes 'K' (abbreviation for the unit of temperature).
'plan'+'c'+'k'='PLANCK'
(Other definitions for planck that I've seen before include "Max ---, quantum theorist" , "Max --, German scientist, gave name to a constant" , "German physicist giving name to a constant" , "German physicist, initiator of quantum theory, d. 1947" .)