Resolute biblical character, leader of army (NT) (7)
I believe the answer is:
adamant
'resolute' is the definition.
(both can mean determined)
'biblical character leader of army nt' is the wordplay.
'biblical character' becomes 'Adam' (in the Book of Genesis).
'leader of' indicates taking the first letters.
The first letter of 'army' is 'a'.
'adam'+'a'+'nt'='ADAMANT'
(Other definitions for adamant that I've seen before include "Positive and unrelenting" , "Unyielding to requests, stubborn" , "Unyielding, won't change mind" , "Legendary rock" , "Positive or intransigent" .)