Darling tenor's intro to Regret Nothing (8)
I believe the answer is:
truelove
'darling' is the definition.
(thesaurus)
'tenor's intro to regret nothing' is the wordplay.
'intro' indicates taking the first letters.
'to regret' becomes 'rue' (synonyms).
'nothing' becomes 'love' (love means zero in tennis).
The initial letter of 'tenors' is 't'.
't'+'rue'+'love'='TRUELOVE'
(Other definitions for truelove that I've seen before include "Quatrefoil" , "''The path of . . . . . . . . ne'er runs smooth''" , "'The course of ..... ne'er runs smooth'" , "dearest" , "'The course of ... ... ne'er ran smooth'" .)