Loathe best man regularly rejected by husband (4)
I believe the answer is:
hate
'loathe' is the definition.
(thesaurus)
'best man regularly rejected by husband' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'rejected' is a reversal indicator.
'by' means one lot of letters go next to another.
'husband' becomes 'h'.
'best'+'man'='bestman'
The alternating letters of 'bestman' are 'eta'.
'eta' reversed gives 'ate'.
'ate' put after 'h' is 'HATE'.
(Other definitions for hate that I've seen before include "Thoroughly dislike, loathe" , "Abhor" , "Strong aversion" , "powerful emotion" , "Abominate" .)