Slug certainly no snail! (6)
I believe the answer is:
bullet
'slug' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bullet that I've seen before include "Gun projectile" , "Piece of ammunition" , "High-speed projectile" , "Lethal projectile" , "Small missile from gun" .)