Pair inside hollow cog (8)
I believe the answer is:
sprocket
'cog' is the definition.
(I know that cog can be written as sprocket)
'pair inside hollow' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'inside' means one lot of letters goes inside another.
'hollow' becomes 'socket' (I've seen this before).
'pr' put within 'socket' is 'SPROCKET'.
(Other definitions for sprocket that I've seen before include "Wheel projection engaging with hole in, eg, film" , "Tooth on the rim of a driving wheel" , "Roller with teeth" , "sort of wheel" , "It helps to drive" .)