Chairs can somehow become a sweetener (9)
I believe the answer is:
saccharin
'a sweetener' is the definition.
(I know that saccharin is a type of sweetener)
'chairs can somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'chairs'+'can'='chairscan'
'chairscan' anagrammed gives 'SACCHARIN'.
'become' acts as a link.
(Other definitions for saccharin that I've seen before include "Calorie-free sweetener" , "sweetener provided" , "alternative sweetener" , "Very sweet compound" , "Artificial sweetener" .)