Mortar ingredient (6)
I believe the answer is:
cement
'mortar ingredient' 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 "Join firmly together" , "It hardens when wetted" , "Powder used to make concrete" , "Ingredient of concrete" , "Material used by builders and dentists" .)