Lingering impression is old and unfriendly (5)
I believe the answer is:
odour
'lingering impression' is the definition.
(an odour might linger)
'old and unfriendly' is the wordplay.
'old' becomes 'o' (abbreviation).
'and' says to put letters next to each other.
'unfriendly' becomes 'dour'.
'o'+'dour'='ODOUR'
'is' is the link.
(Other definitions for odour that I've seen before include "reputation" , "A smell, good or bad" , "Perfume" , "for the nose to detect" , "Aroma, smell" .)