Monumental bit of stonework that's built up on Hampstead Heath (6)
I believe the answer is:
plaque
This clue is a double definition.
'monumental bit of stonework' is the first definition.
(a plaque is a stone monument)
'that's built up on hampstead heath' is the second definition.
(Hampstead Heath is rhyming slang for teeth)
(Other definitions for plaque that I've seen before include "Substance that forms on teeth encouraging bacteria" , "Film on tooth" , "Unhygienic substance that forms on teeth" , "Dental coating" , "Memorial slab" .)