Spook agent receiving shocking treatment having landed by river (11)

'river' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

'spook agent receiving shocking treatment having landed' is the wordplay.
'spook agent' becomes 'spry' (I can't justify this - if you can you should give a lot more credence to this answer).
'receiving' is an insertion indicator.
'shocking treatment' becomes 'ect' (electro-convulsive therapy).
'having' means one lot of letters goes inside another.
'landed' becomes 'alit' (I've seen this before).
'spry' enclosing 'ect' is 'spectry'.
'spectry' enclosing 'alit' is 'SPECTRALITY'.

'by' acts as a link.

Can you help me to learn more?