Obscure bit of chemistry (4)
I believe the answer is:
mist
'obscure' is the definition.
('mist' can be a synonym of 'obscure')
'bit of chemistry' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'MIST' is hidden within 'chemistry'.
(Other definitions for mist that I've seen before include "Haze" , "Fret" , "Watery film" , "Think fog or low cloud" , "Mizzle; light rain" .)