Key argument in legal document (6)

I believe the answer is:

escrow

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Here is my best explanation:

'legal document' is the definition.
(escrow is a kind of legal document)

'key argument' is the subsidiary indication.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'argument' becomes 'row' (I've seen this before).
'esc'+'row'='escrow'

'in' is the link.

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This clue was last seen in The Independent

(Other definitions for escrow that I've seen before include "It's held in trust or as a security", "Legal instrument", "Deed held by third party effective when a condition is met", "conditional execution of deed", "back-up security", "goods held by third party", "Conditional deed held by third party", "fund held in trust?", "Legal deed to take effect when a condition is fulfilled" and "document held by third party".)

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