Vowed at first she would only remarry nervously (5)
I believe the answer is:
sworn
'vowed' is the definition.
(thesaurus)
'first she would only remarry nervously' is the wordplay.
'first' says to take the initial letters.
The initial letters of 'she would only remarry nervously' is 'SWORN'.
'at' acts as a link.
(Other definitions for sworn that I've seen before include "confirmed in office" , "Given under oath" , "promised" , "Affirmed" , "Stated on oath" .)