Initially tests resolve over treacherous husband's fidelity (5)
I believe the answer is:
troth
'fidelity' is the definition.
(thesaurus)
'initially tests resolve over treacherous husband's' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'tests resolve over treacherous husbands' is 'TROTH'.
(Other definitions for troth that I've seen before include "Something often plighted" , "Pledge of fidelity" , "old-fashioned faith" , "Solemn word" , "Solemn pledge" .)