Old snack: smell it coming up (6)
I believe the answer is:
tiffin
'old snack' is the definition.
(chocolate bar no longer sold)
'smell it coming up' is the wordplay.
'smell' becomes 'niff' (niff is a kind of smell).
'coming up' says the letters should be written in reverse.
'niff'+'it'='niffit'
'niffit' written backwards gives 'TIFFIN'.
(Other definitions for tiffin that I've seen before include "Light midday meal in India" , "Snack, light meal (dated)" , "Light Indian repast" , "Midday meal for members of the British Raj in India" , "(Anglo Indian) lunch" .)