Key argument in legal document (6)

I believe the answer is:
escrow
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.
(Other definitions for escrow that I've seen before include "Conditional deed held by third party", "It's held until a condition is fulfilled", "Money held against completion", "Deed held by third party effective when a condition is met", "Legal deed to take effect when a condition is fulfilled", "Deed or deposit held until a condition is fulfilled", "It's held in trust or as a security" and "Something held until a condition is fulfilled".)

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