Common condition among soldiers (4)
I believe the answer is:
rife
'common' is the definition.
(thesaurus)
'condition among soldiers' is the wordplay.
'condition' becomes 'if' (I've seen this in other clues).
'among' indicates putting letters inside.
'soldiers' becomes 're' (Royal Engineers).
'if' going within 're' is 'RIFE'.
(Other definitions for rife that I've seen before include "Of widespread occurrence" , "Very common or frequently occurring" , "Widespread sort of fire" , "Prevalent" , "Overabundant" .)