One senses them being so backward, gloomy inside (6)
I believe the answer is:
odours
'one senses them' is the definition.
(you sense odours with your nose)
'so backward gloomy inside' is the wordplay.
'backward' says the letters should be written in reverse.
'gloomy' becomes 'dour' (I've seen this before).
'inside' indicates putting letters inside.
'so' in reverse letter order is 'os'.
'os' enclosing 'dour' is 'ODOURS'.
'being' acts as a link.
(Other definitions for odours that I've seen before include "bouquets" , "Smells of any kind" , "emanations" , "Distinctive smells" , "Smells, usually unpleasant" .)