Sodium smell is beginning to develop (7)
I believe the answer is:
nascent
'beginning to develop' is the definition.
(I've seen this before)
'sodium smell' is the wordplay.
'sodium' becomes 'na' (chemical symbol).
'smell' becomes 'scent' (scent is a kind of smell).
'na'+'scent'='NASCENT'
'is' acts as a link.
(Other definitions for nascent that I've seen before include "starting to happen" , "Dawning" , "being developed" , "Being born - scan net (anag)" , "in early stages" .)