Need to immerse sir in river (6)

I believe the answer is:

desire

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'need' is the definition.
(I have seen 'Strong need ' mean 'desire' so perhaps 'need' could also mean 'desire')

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for desire that I've seen before include "Longing or wish" , "Famous play: ''A Streetcar Named ......''" , "Strong wish" , "Reside (anag.)" , "Hanker after" .)

I've seen this clue in The Sun.

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