Small stone lodged in baby tooth (8)
I believe the answer is:
sprocket
'tooth' is the definition.
(I know that sprocket is a type of tooth)
'small stone lodged in baby' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stone' becomes 'rock' (synonyms).
'lodged in' is an insertion indicator.
'baby' becomes 'pet'.
'rock' placed within 'pet' is 'procket'.
's'+'procket'='SPROCKET'
(Other definitions for sprocket that I've seen before include "Tooth for driving mechanism" , "Something like gear" , "Wheel with projecting teeth" , "Series of teeth on wheel engaging with chain" , "sort of wheel" .)