Gloomy surrounded by very big smells (6)
I believe the answer is:
odours
'smells' is the definition.
('odour' can be a synonym of 'smell')
'gloomy surrounded by very big' is the wordplay.
'gloomy' becomes 'dour' (I've seen this before).
'surrounded by' is an insertion indicator.
'very big' becomes 'os' (abbreviation for outsize).
'dour' going inside 'os' is 'ODOURS'.
(Other definitions for odours that I've seen before include "Smells and fragrances" , "bouquets" , "Smells of any kind" , "for the nose to detect" , "Smells, good or bad" .)