Popular smell discovered by a northern savage (7)
I believe the answer is:
inhuman
'savage' is the definition.
(I've seen this in another clue)
'popular smell discovered by a northern' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'smell' becomes 'hum' (something with a bad smell is 'humming').
'discovered by' means one lot of letters go next to another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'in'+'hum'+'a'+'n'='INHUMAN'
(Other definitions for inhuman that I've seen before include "barbarous and brutal" , "Beastly" , "diabolical ..." , "Nasty" , "Without compunction" .)