Decay surrounding old part of tooth (4)
I believe the answer is:
root
'part of tooth' is the definition.
('root' is part of 'tooth')
'decay surrounding old' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'surrounding' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rot' placed around 'o' is 'ROOT'.
(Other definitions for root that I've seen before include "Origin; carrot perhaps" , "top of tree? Hardly!" , "Underground part of a plant" , "essential nature" , "Origin - stem" .)