The most common and most likely answer to this crossword clue is the 6 plus 5 letter word PURPLE HEART
P
U
R
P
L
E
H
E
A
R
T
Buddy explains!
The straight part
We know that "Injured GI could be given this/drug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case injured gi could be given this/drug = purple heart.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.