What does the cryptic crossword clue "Battle in progress, no messing up! (4)" mean?

The definition is "Battle". The wordplay uses hidden reversed. Hidden reversed in "battle in progresS, NO Messing up!" The answer is Mons.

Battle in progress, no messing up! (4)

Telegraph Prize Cryptic #3098 | 1 Down | 2021-03-07 flower