Firmness of purpose shown by Will (13)
I believe the answer is:
determination
I believe this clue is a double definition.
'firmness of purpose' is the first definition.
(I've seen this before)
'will' is the second definition.
(thesaurus)
'shown by' is the link.
(Other definitions for determination that I've seen before include "Firm resolve" , "Decision process" , "Resolution" , "Perseverance" , "legal decision" .)