I've an order that's simple (5)
I believe the answer is:
naive
'that's simple' is the definition.
(similar in meaning)
'i've an order' is the wordplay.
'order' is an anagram indicator.
'ive'+'an'='ivean'
'ivean' with letters rearranged gives 'NAIVE'.
(Other definitions for naive that I've seen before include "childlike" , "Unaffected; credulous" , "Simple, innocent or unsophisticated" , "Lacking wisdom and judgement" , "Lacking sophistication" .)