Projectile taking top off mound (5)
I believe the answer is:
arrow
'projectile' is the definition.
(arrow is a kind of projectile)
'taking top off mound' is the wordplay.
'taking top off' means to remove the first letter.
'mound' becomes 'barrow' (barrow is a kind of mound).
'barrow' with its first letter taken off is 'ARROW'.
(Other definitions for arrow that I've seen before include "Early guided missile" , "Type of projectile" , "Mark with pointer" , "marker" , "Missile fired from bow" .)