First male soldier is stubborn (7)
I believe the answer is:
adamant
'stubborn' is the definition.
(thesaurus)
'first male soldier' is the wordplay.
'first male' becomes 'adam'.
'soldier' becomes 'ant' (soldier is a type of ant).
'adam'+'ant'='ADAMANT'
'is' acts as a link.
(Other definitions for adamant that I've seen before include "Refusing to budge" , "Impervious to pleas or persuasion" , "Refusing to change one's mind" , "With mind made up" , "Hard stone" .)