At the outset, Tories regret nothing, darling (8)
I believe the answer is:
truelove
'darling' is the definition.
(thesaurus)
'at the outset tories regret nothing' is the wordplay.
'at the outset' suggests taking the first letters (the outset is the beginning).
'regret' becomes 'rue' (to rue something is to regret it).
'nothing' becomes 'love' (love means zero in tennis).
The first letter of 'tories' is 't'.
't'+'rue'+'love'='TRUELOVE'
(Other definitions for truelove that I've seen before include "'The course of ... ... ne'er ran smooth'" , "Quatrefoil" , "''The path of . . . . . . . . ne'er runs smooth''" , "'The course of ..... ne'er runs smooth'" , "dearest" .)