Bit of tooth decay multiplied internally (4)
I believe the answer is:
root
'bit of tooth' is the definition.
('root' is part of 'tooth')
'decay multiplied internally' is the wordplay.
'decay' becomes 'rot' (rot can mean to decay).
'multiplied' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'internally' means one lot of letters goes inside another.
'rot' enclosing 'o' is 'ROOT'.
Can you help me to learn more?
(Other definitions for root that I've seen before include "Reptile" , "Lowest part of plant" , "grub" , "Source; firmly establish" , "radical" .)