The old soldier's first, of course (3)
I believe the answer is:
yes
'of course' is the definition.
(I've seen this in another clue)
'the old soldier's first' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'soldier's first' becomes 's' (1st letter of 'soldier').
'ye'+'s'='YES'
(Other definitions for yes that I've seen before include "Affirmative response" , "Affirmative reply" , "Yea" , "Validating vote" , "Of course" .)