Battle in progress, no messing up! (4)
I believe the answer is:
mons
'battle' is the definition.
(I've seen this before)
'in progress no messing up' is the wordplay.
'in' indicates a hidden word.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'snom' can be found hidden inside 'progress no messing'.
'snom' reversed gives 'MONS'.
(Other definitions for mons that I've seen before include "place of conflict?" , "Belgian World War One battle site" , "Battlefield" , "Belgian city; WW1 battle" , "Belgian town" .)