Soldiers find expert a nuisance (6)
I believe the answer is:
menace
'a nuisance' is the definition.
(I have seen 'serious nuisance' mean 'menace' so perhaps 'nuisance' could also mean 'menace')
'soldiers find expert' is the wordplay.
'soldiers' becomes 'men' (I've seen this before).
'find' is a charade indicator (letters next to each other).
'expert' becomes 'ace' (ace is a kind of expert).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threaten with harm or danger" , "Act in a threatening manner" , "peril" , "serious nuisance" , "Serious threat" .)