Key line in legal arrangement (6)
I believe the answer is:
escrow
'legal arrangement' is the definition.
(escrow is a type of legal agreement)
'key line' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'line' becomes 'row' (a row or line of things).
'esc'+'row'='ESCROW'
'in' acts as a link.
(Other definitions for escrow that I've seen before include "Legal deed to take effect when a condition is fulfilled" , "It's held in trust or as a security" , "Deed or deposit held until a condition is fulfilled" , "Money held against completion" , "Something held until a condition is fulfilled" .)