Decision involving strength of willpower (10)
I believe the answer is:
resolution
This clue is a triple definition!
'decision' is the first definition.
(I know that resolution is a type of decision)
'involving' is a link.
'strength' is the second definition.
(thesaurus)
'of' is a link.
'willpower' is the third definition.
(I know that willpower can be written as resolution)
(Other definitions for resolution that I've seen before include "Proposal" , "puts an end to discord" , "Degree of detail" , "Verdict" , "Firm decision" .)