Optimistic investor, extremely efficient, getting sack (6)
I believe the answer is:
bullet
'sack' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'optimistic investor extremely efficient getting' is the wordplay.
'optimistic investor' becomes 'bull' (bull is a kind of investor. I am not sure about the 'optimistic' bit.).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'getting' is a charade indicator (letters next to each other).
'efficient' with its centre taken out is 'et'.
'bull'+'et'='BULLET'
Can you help me to learn more?
(Other definitions for bullet that I've seen before include "Gun projectile" , "Slug" , "Point in report" , "Lethal projectile" , "Small missile from gun" .)