Unpleasant smell: a new kind (5)
I believe the answer is:
human
'kind' is the definition.
(thesaurus)
'unpleasant smell a new' is the wordplay.
'unpleasant smell' becomes 'hum' (hum can mean a disgusting smell).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hum'+'a'+'n'='HUMAN'
(Other definitions for human that I've seen before include "likely to err?" , "Being that's apt to err?" , "Man, woman or child" , "Like a real person" , "Characteristic of people" .)