Courageous, crossing line for sweetheart (7)
I believe the answer is:
darling
'sweetheart' is the definition.
(thesaurus)
'courageous crossing line' is the wordplay.
'courageous' becomes 'daring' (I've seen this before).
'crossing' indicates putting letters inside.
'line' becomes 'l'.
'daring' enclosing 'l' is 'DARLING'.
'for' acts as a link.
(Other definitions for darling that I've seen before include "Adorable - honeybun" , "Term of endearment" , "Australian tributary of the Murray River, sweetheart" , "Beloved" , "Victorian heroine" .)