Key line in legal contract (6)
I believe the answer is:
escrow
'legal contract' is the definition.
'key line' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'line' becomes 'row' (row is a kind of line).
'esc'+'row'='ESCROW'
'in' is the link.
(Other definitions for escrow that I've seen before include "fund held in trust?" , "goods held by third party" , "Legal deed to take effect when a condition is fulfilled" , "Money held against completion" , "Conditional deed held by third party" .)