Troublesome guerrilla army oddly ignored (8)
I believe the answer is:
contrary
'troublesome' is the definition.
(thesaurus)
'guerrilla army oddly ignored' is the wordplay.
'guerrilla' becomes 'contra' (the Contras were Nicaraguan guerrilla fighters).
'oddly ignored' means one should take alternating letters.
The alternating letters of 'army' are 'ry'.
'contra'+'ry'='CONTRARY'
(Other definitions for contrary that I've seen before include "Opposite, perverse" , "Opposed or perverse" , "Exact opposition" , "At variance; awkward" , "Carry out (command)" .)