Line added to US soldier’s very short book (4)
I believe the answer is:
joel
'very short book' is the definition.
(short book of the Bible)
'line added to us soldier's' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'added to' means one lot of letters go next to another.
'us soldier' becomes 'Joe' (GI Joe can mean American soldier).
'l' after 'joe' is 'JOEL'.
(Other definitions for joel that I've seen before include "Book of minor prophet" , "Old Testament prophet" , "book in church" .)