Adversary’s force finally upset Yankee soldiers (5)
I believe the answer is:
enemy
'adversary's' is the definition.
(thesaurus)
'force finally upset yankee soldiers' is the wordplay.
'finally' indicates one should take the final letters.
'upset' says the letters should be written in reverse.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'soldiers' becomes 'men' (I've seen this before**).
The final letter of 'force' is 'e'.
'y'+'men'='ymen'
'ymen' written backwards gives 'nemy'.
'e'+'nemy'='ENEMY'
(Other definitions for enemy that I've seen before include "Yemen (anag.)" , "Time" , "Division" , "is against us" , "Opposing force" .)