Post tablet a doctor finally supplies (6)
I believe the answer is:
pillar
'post tablet a doctor finally supplies' is the definition.
I can't tell whether this definition defines the answer.
'post tablet a doctor finally' is the wordplay.
'post' means one lot of letters go next to another (post- can mean after).
'tablet' becomes 'pill' (synonyms).
'finally' indicates one should take the final letters.
The final letter of 'doctor' is 'r'.
'pill'+'a' is 'pilla'.
'pilla'+'r'='PILLAR'
(Other definitions for pillar that I've seen before include "Support structure" , "Upright post" , "Architectural support" , "Building support" , "Supporter" .)