Key argument for legal arrangement (6)
I believe the answer is:
escrow
'legal arrangement' is the definition.
(escrow is a type of legal agreement)
'key argument' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'argument' becomes 'row' (to row is to argue).
'esc'+'row'='ESCROW'
'for' acts as a link.
(Other definitions for escrow that I've seen before include "document held by third party" , "Money held against completion" , "Conditional deed held by third party" , "conditional execution of deed" , "Deed or deposit held until a condition is fulfilled" .)