Key argument in legal document (6)
I believe the answer is:
escrow
'legal document' is the definition.
(escrow is a kind of legal document)
'key argument' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'argument' becomes 'row' (I've seen this before).
'esc'+'row'='ESCROW'
'in' acts as a link.
(Other definitions for escrow that I've seen before include "fund held in trust?" , "Money held against completion" , "Deed held by third party effective when a condition is met" , "document held by third party" , "back-up security" .)