Slender new missile (6)
I believe the answer is:
narrow
'slender' is the definition.
(similar in meaning)
'new missile' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'missile' becomes 'arrow' (arrow is a kind of missile).
'n'+'arrow'='NARROW'
(Other definitions for narrow that I've seen before include "Small-minded" , "Restrict" , "Thin; taper" , "Bigoted opinion" , "Keen" .)