The most common and most likely answer to this crossword clue is the 5 letter word ENROL
I've seen this crossword clue in The Times
E
N
R
O
L
More clues leading to the result ENROL
- Record single up around number four in chart
- Register of leaderless soldiers with inadequate function
- Loner forced to join up
- Revolutionary wisdom gathered about new recruit
- Reformed loner to join society?
- Record single rising — around number four in chart
- In from France given theatrical part, we hear, register for course
- Register new part, the last to be taken up
- Join in French function that's endless
- Loner ordered to join army?
Buddy explains!

The straight part
We know that "join army" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case join army = enrol.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.
What does ENROL mean?
ENROL verb- To enter (a name, etc.) in a register, roll or list
- To enlist (someone) or make (someone) a member of
- To enlist oneself (in something) or become a member (of something)
- To envelop; to enwrap.