Remains very unwell visiting north-eastern US city (9)
I believe the answer is:
'us city' is the definition.
(I've seen this before)
'remains very unwell visiting north-eastern' is the wordplay.
'remains' becomes 'ash' (remains of a fire).
'very' becomes 'v' (common abbreviation).
'unwell' becomes 'i'll' (similar in meaning).
'visiting' is an insertion indicator.
'north eastern' becomes 'NE' (abbreviation).
'ashvill' going inside 'ne' is 'NASHVILLE'.
(Other definitions for nashville that I've seen before include "country town" , "music centre" , "Grand Ole Opry city" , "Centre of country and western" , "US city" .)