Stupidly volunteer, forsaking new sweetheart
I believe the answer is:
truelove
'sweetheart' is the definition.
(synonyms)
'stupidly volunteer forsaking new' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'forsaking' is a deletion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'volunteer' with 'n' taken out is 'voluteer'.
'voluteer' anagrammed gives 'TRUELOVE'.
(Other definitions for truelove that I've seen before include "Genuine affection?" , "dearest" , "'The course of ..... ne'er runs smooth'" , "Sweetheart" , "'The course of ... ... ne'er ran smooth'" .)