Naughty Kelvin leaves a lot of washing-up (6)
I believe the answer is:
sinful
'naughty' is the definition.
(thesaurus)
'kelvin leaves a lot of washing-up' is the wordplay.
'kelvin' becomes 'k' (abbreviation for the unit of temperature).
'leaves' suggests deleting specific letters.
'a lot of washing-up' becomes 'sinkful'.
'sinkful' with 'k' taken away is 'SINFUL'.
(Other definitions for sinful that I've seen before include "Habitually wicked" , "Wicked, unholy" , "Offensive to God" , "Morally wrong; wicked" , "Fragrant climber" .)