Army head acquires a second scar (9)
I believe the answer is:
cicatrice
'scar' is the definition.
(I know that scar can be written as cicatrice)
'army head acquires a second' is the wordplay.
'army head' becomes 'CIC' (abbreviation for Commander-in-Chief).
'acquires' means one lot of letters go next to another.
'second' becomes 'trice' (both can mean a very short amount of time).
'cic'+'a'+'trice'='CICATRICE'
(Other definitions for cicatrice that I've seen before include "something over wound" , "Scar over a healed wound" .)