Cannoli, essentially ground stuff in a shell (5)
I believe the answer is:
nacre
'shell' is the definition.
(nacre is a kind of shell)
'cannoli essentially ground stuff in a' is the wordplay.
'cannoli' becomes 'ncr' (I can't explain this - if you can you should give a lot more credence to this answer).
'essentially' indicates the central letters.
'ground' becomes 'rec' (I've seen this before).
'stuff in' means one lot of letters goes inside another.
The middle letter of 'rec' is 'e'.
'ncr'+'e'='ncre'
'ncre' enclosing 'a' is 'NACRE'.
Can you help me to learn more?
(Other definitions for nacre that I've seen before include "Mother-of-pearl - crane (anag)" , "inside of shell" , "Mother-of-pearl from shellfish" , "Pearl's mum" , "Mother of pearl" .)