Four breaking oath joined up (7)
I believe the answer is:
cursive
'joined up' is the definition.
'four breaking oath' is the wordplay.
'four' becomes 'iv' ('IV' can be a synonym of 'four').
'breaking' is an insertion indicator.
'oath' becomes 'curse' ('curse' can be a synonym of 'oath').
'iv' inserted within 'curse' is 'CURSIVE'.
(Other definitions for cursive that I've seen before include "Joined-up (writing)" , "script" .)