Chlorine's always sharp (6)
I believe the answer is:
clever
'sharp' is the definition.
(I know that clever is a more specific form of the attribute sharp)
'chlorine's always' is the wordplay.
'chlorine' becomes 'Cl' (chemical symbol).
'always' becomes 'ever' (synonyms).
'cl'+'ever'='CLEVER'
(Other definitions for clever that I've seen before include "Box ---, outwit someone" , "Cute" , "Good or quick at learning" , "Knowledgeable" , "Brainy, smart" .)