Dutchman's pipe in long drain (7)

I believe the answer is:

pinesap

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'dutchman's pipe' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'long drain' is the wordplay.
'long' becomes 'pine' (to pine for someone is to long for them).
'drain' becomes 'sap' (sap can mean to suck or drain).
'pine'+'sap'='PINESAP'

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in The Times.

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