Favourite line put in bold (7)
I believe the answer is:
darling
'favourite' is the definition.
('darling' can be a synonym of 'favourite')
'line put in bold' is the wordplay.
'line' becomes 'l'.
'put in' means one lot of letters goes inside another.
'bold' becomes 'daring' (I've seen this before).
'l' inserted into 'daring' is 'DARLING'.
(Other definitions for darling that I've seen before include "Australian river and loved one" , "Australian tributary of the Murray River, sweetheart" , "Victorian heroine" , "Grace -, nineteenth century wreck heroine of lighthouse" , "Film with Julie Christie and Dirk Bogarde - sweetheart" .)