Decay around centre of tooth’s base (4)
I believe the answer is:
root
'base' is the definition.
(synonyms)
'decay around centre of tooth's' is the wordplay.
'decay' becomes 'rot' (rotting is a kind of decaying).
'around' indicates putting letters inside.
'centre of' means to look at the middle letters.
The centre of 'tooth' is 'o'.
'rot' going around 'o' is 'ROOT'.
(Other definitions for root that I've seen before include "burrow" , "essential nature" , "Source; firmly establish" , "Basic source or origin" , "top of tree? Hardly!" .)