Right-o, sweetheart! (8)
I believe the answer is:
truelove
'sweetheart' is the definition.
('truelove' can be a synonym of 'sweetheart')
'right-o' is the wordplay.
'right' becomes 'true' ('true' is associated in meaning with 'right').
'o' becomes 'love' (I've seen this in other clues).
'true'+'love'='TRUELOVE'
(Other definitions for truelove that I've seen before include "Genuine affection?" , "'The course of ..... ne'er runs smooth'" , "Sweetheart" , "''The path of . . . . . . . . ne'er runs smooth''" , "Quatrefoil" .)