Soldiers and sergeant finally in action, barking (8)
I believe the answer is:
demented
'barking' is the definition.
'soldiers and sergeant finally in action' is the wordplay.
'soldiers' becomes 'men' (I've seen this before**).
'and' says to put letters next to each other.
'finally' says to take the final letters.
'in' means one lot of letters goes inside another.
'action' becomes 'deed' (deed is a kind of action).
The last letter of 'sergeant' is 't'.
'men'+'t'='ment'
'ment' going within 'deed' is 'DEMENTED'.
(Other definitions for demented that I've seen before include "Wild, irrational" , "Crazy, insane" , "Nutty" , "Deranged, crazy" , "Out of one's mind" .)