Brave, crossing line for sweetheart (7)
I believe the answer is:
darling
'for sweetheart' is the definition.
(both are terms of address to romantic partner)
'brave crossing line' is the wordplay.
'brave' becomes 'daring' (daring can mean adventurous or courageous).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'line' becomes 'l' (used when specifying particular lines from a poem).
'daring' going around 'l' is 'DARLING'.
(Other definitions for darling that I've seen before include "Adorable - honeybun" , "Term of affection" , "Term of endearment" , "Beloved" , "Australian tributary of the Murray River, sweetheart" .)