This diary is not, oddly, about people (5)
I believe the answer is:
irish
'people' is the definition.
(the Irish are the people of Ireland)
'this diary is not oddly about' is the wordplay.
'not oddly' means one should take alternating letters (odd letters are removed).
'about' shows that the letters should be reversed in order.
'this'+'diary'+'is'='thisdiaryis'
The alternate letters of 'thisdiaryis' are 'hsiri'.
'hsiri' written backwards gives 'IRISH'.
(Other definitions for irish that I've seen before include "Joyce was" , "Reddish-brown dog" , "Eg, native to Dublin" , "Of this place" , "A people, language and whiskey" .)