Bitterness shown by Army CO in trouble (8)
I believe the answer is:
acrimony
'bitterness' is the definition.
(I know that bitterness can be written as acrimony)
'army co in trouble' is the wordplay.
'trouble' is an anagram indicator.
'army'+'co'+'in'='armycoin'
'armycoin' is an anagram of 'ACRIMONY'.
'shown by' acts as a link.
(Other definitions for acrimony that I've seen before include "Bitterness of manner or words" , "Acronym I construct with bitter words" , "Bitterness of temper" , "Bitterness, ill feeling" , "'Bitterness, sharpness (8)'" .)