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 "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".)