Slender tip of Neolithic projectile (6)
I believe the answer is:
narrow
'slender' is the definition.
('narrow' can be similar in meaning to 'slender')
'tip of neolithic projectile' is the wordplay.
'tip of' indicates taking the first letters.
'projectile' becomes 'arrow' (arrow is a kind of projectile).
The first letter of 'neolithic' is 'n'.
'n'+'arrow'='NARROW'
(Other definitions for narrow that I've seen before include "Short of breadth" , "Small-minded" , "Cramped" , "lacks vision" , "... not one" .)