Forces one drunk into beaten army (8)
I believe the answer is:
military
'forces' is the definition.
(I have seen 'Armed forces ' mean 'military' so perhaps 'forces' could also mean 'military')
'one drunk into beaten army' is the wordplay.
'one' becomes 'i' (Roman numeral).
'drunk' becomes 'lit' (I've seen this in another clue).
'into' means one lot of letters goes inside another.
'beaten' indicates an anagram (letters beaten up or destroyed).
'i'+'lit'='ilit'
'army' with letters rearranged gives 'mary'.
'ilit' going inside 'mary' is 'MILITARY'.
(Other definitions for military that I've seen before include "Relating to soldiers or war" , "Of soldiers and warfare" , "Relating to the army" , "Related to soldiering" , "soldierly" .)