The most common and most likely answer to this crossword clue is the 3 plus 4 letter word EGG COSY
I've seen this crossword clue in Financial Times
E
G
G
C
O
S
Y
Buddy explains!
The straight part
We know that "What keeps the place warm before soldiers enter?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what keeps the place warm before soldiers enter? = egg cosy.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.