Favourite line, in bold (7)
I believe the answer is:
darling
'favourite' is the definition.
('darling' can be a synonym of 'favourite')
'line in bold' is the wordplay.
'line' becomes 'l'.
'in' means one lot of letters goes inside another.
'bold' becomes 'daring' (I've seen this before).
'l' inserted inside 'daring' is 'DARLING'.
(Other definitions for darling that I've seen before include "Film with Julie Christie and Dirk Bogarde - sweetheart" , "Australian tributary of the Murray River, sweetheart" , "Term of endearment" , "lover" , "Adorable - honeybun" .)