Chief of police at grain store, certainly disheartened to find container holding hallucinogenic substance (10)

'hallucinogenic substance' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'chief of police at grain store certainly disheartened to find container' is the wordplay.
'chief of' suggests taking the first letters.
'at' says to put letters next to each other.
'grain store' becomes 'silo' (I've seen this before).
'disheartened' means to remove the middle letters (the middle or heart is removed).
'to find container' becomes 'bin' (I can't justify this - if you can you should give a lot more credence to this answer).
'certainly' with its middle taken out is 'cy'.
The first letter of 'police' is 'p'.
'p'+'silo'+'cy'+'bin'='PSILOCYBIN'

'holding' acts as a link.

Can you help me to learn more?