To a doctor a precious thing is the last tablet (9)
I believe the answer is:
'the last tablet' is the definition.
'tombstone' can be an answer for 'tablet' (thesaurus). I'm unsure of the 'the last' bit.
'to a doctor a precious thing' is the wordplay.
'a doctor' becomes 'MB' (Bachelor of Medicine).
'a precious thing' becomes 'stone' (I can't explain this - if you can you should give a lot more credence to this answer).
'is' is the link.
Can you help me to learn more?
(Other definitions for tombstone that I've seen before include "Where to find epitaph" , "Graveyard marker" , "Last monument" , "film location" , "Does it display departure date" .)