Good disposal of heat, perhaps, dropping one degree (9)
I believe the answer is:
gradation
'degree' is the definition.
(thesaurus)
'good disposal of heat perhaps dropping one' is the wordplay.
'good' becomes 'g' (abbreviation).
'disposal of heat perhaps' becomes 'radiation'.
'dropping' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'radiation' with 'i' taken away is 'radation'.
'g'+'radation'='GRADATION'
(Other definitions for gradation that I've seen before include "Degree, step" , "Change of tone" , "Spoil" , "Scale of successive changes or stages" , "series of changes" .)