Encrustation to mount (5)
I believe the answer is:
scale
I believe this clue is a double definition.
'encrustation' is the first definition.
(thesaurus)
'mount' is the second definition.
(scaling is a kind of mounting)
'to' is the link.
(Other definitions for scale that I've seen before include "Relative magnitude" , "Climb - balance" , "Size ratio; skin flake" , "Climb; overlapping plate" , "range" .)