Bonding agent (4)
(Other definitions for glue that I've seen before include "size" , "Bond" , "Fixative" , "Gum" , "Paste" .)
'bonding agent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cement that I've seen before include "Bind firmly" , "Firmly establish" , "bind together" , "Bind (friendship)" , "Powder used to make concrete" .)