Slug, animal, extremely efficient (6)
I believe the answer is:
bullet
'slug' is the definition.
(synonyms)
'animal extremely efficient' is the wordplay.
'animal' becomes 'bull' (bull is a kind of animal).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'efficient' with its middle removed is 'et'.
'bull'+'et'='BULLET'
(Other definitions for bullet that I've seen before include "Projectile ammunition" , "Slug" , "Point in report" , "Small missile from gun" , "Piece of ammunition" .)