Bold line taken on for sweetheart (7)
I believe the answer is:
darling
'for sweetheart' is the definition.
(both are terms of address to romantic partner)
'bold line taken' is the wordplay.
'bold' becomes 'daring' (I've seen this before).
'line' becomes 'l' (used when specifying particular lines from a poem).
'taken' is an insertion indicator.
'daring' enclosing 'l' is 'DARLING'.
'on' is the link.
(Other definitions for darling that I've seen before include "family in play" , "Australian river and loved one" , "Australian tributary of the Murray River, sweetheart" , "Beloved" , "Wendy's family (Peter Pan)" .)