Trial I organised in my army (8)
I believe the answer is:
military
'army' is the definition.
(thesaurus)
'trial i organised in my' is the wordplay.
'organised' indicates an anagram.
'in' means one lot of letters goes inside another.
'trial'+'i'='triali'
'triali' anagrammed gives 'ilitar'.
'ilitar' put inside 'my' is 'MILITARY'.
(Other definitions for military that I've seen before include "Relating to armed forces" , "in the field" , "Of soldiers and warfare" , "Relating to soldiers or war" , "Related to soldiering" .)