Refer to the page on drainage! (7)

I believe the answer is:

seepage

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

'refer to the page on' is the wordplay.
'refer to' becomes 'see'.
'on' says to put letters next to each other.
'see'+'page'='SEEPAGE'

Can you help me to learn more?

(Other definitions for seepage that I've seen before include "Slow flow of fluid" , "Slow leaking flow" , "Discharge" , "Loss from slow leak" , "Leaking liquid" .)

I've seen this clue in the Evening Standard.

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