Cart doesn't start as missile (5)
I believe the answer is:
arrow
'missile' is the definition.
(arrow is a kind of missile)
'cart doesn't start' is the wordplay.
'cart' becomes 'barrow' (barrow is a kind of cart).
'doesn't start' means to remove the first letter.
'barrow' with its initial letter taken off is 'ARROW'.
'as' is the link.
(Other definitions for arrow that I've seen before include "Missile with a point" , "Move swiftly" , "production of Fletcher?" , "Direction indicator" , "what may have killed Harold?" .)