The most common and most likely answer to this crossword clue is the 7 plus 7 letter word PRIVATE SOLDIER
I've seen this crossword clue in The Times
P
R
I
V
A
T
E
S
O
L
D
I
E
R
Buddy explains!
The straight part
We know that "Junior in service" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case junior in service = private soldier.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.