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 believe this answer much more).
'essentially' indicates the central letters.
'ground' becomes 'fen' (fen is a kind of ground).
'stuff in' is an insertion indicator.
The middle letter of 'fen' is 'e'.
'ncr'+'e'='ncre'
'ncre' placed around '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" .)