Sharp or caustic (5)
I believe the answer is:
acrid
This clue is a double definition.
'sharp' is the first definition.
(I've seen this before)
'caustic' is the second definition.
(I know that caustic can be written as acrid)
(Other definitions for acrid that I've seen before include "Painfully pungent" , "irritating" , "Bitter in smell, like smoke" , "Greek goddess" , "Strong and sharp-smelling" .)