Extremely envious about line in legal arrangement (6)
I believe the answer is:
escrow
'legal arrangement' is the definition.
(escrow is a type of legal agreement)
'extremely envious about line' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'about' becomes 'c' (circa).
'line' becomes 'row' (a row or line of things).
'envious' with its centre taken out is 'es'.
'es'+'c'+'row'='ESCROW'
'in' acts as a link.
(Other definitions for escrow that I've seen before include "Legal instrument" , "It's held in trust or as a security" , "back-up security" , "Deed held by third party effective when a condition is met" , "fund held in trust?" .)