The most common and most likely answer to this crossword clue is the 3 plus 5 letter word USE FORCE
I've seen this crossword clue in The Guardian
U
S
E
F
O
R
C
E
More clues leading to the result USE FORCE
Buddy explains!
The straight part
We know that "to send in troops" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to send in troops = use force.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.