Size to reduce in confectionery item (7)
I believe the answer is:
gumdrop
'confectionery item' is the definition.
(type of sweet)
'size to reduce' is the wordplay.
'size' becomes 'gum' (both can mean a sticky solution).
'to reduce' becomes 'drop' (both can mean to decrease).
'gum'+'drop'='GUMDROP'
'in' is the link.
(Other definitions for gumdrop that I've seen before include "chew" , "Gelatinous type of sweet" , "Firm jelly sweet" , "Small sweet (goody goody!)" , "Type of jelly sweet" .)