Wheel, let me add, turning on missile (8)
I believe the answer is:
sprocket
'wheel' is the definition.
(sprocket is a kind of wheel)
'let me add turning on missile' is the wordplay.
'let me add' becomes 'ps' (postscript).
'turning' shows that the letters should be reversed in order.
'on' says to put letters next to each other.
'missile' becomes 'rocket'.
'ps' backwards is 'sp'.
'sp'+'rocket'='SPROCKET'
(Other definitions for sprocket that I've seen before include "Wheel projection engaging with hole in, eg, film" , "Thin wheel with teeth driven by a chain" , "Toothed wheel that interlocks with a chain" , "Tooth for driving mechanism" , "Roller with teeth" .)