Key argument in legal document (6)
I believe the answer is:
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).
'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".)