Categorised group on condition that one is a journalist (10)
I believe the answer is:
classified
'categorised group' is the definition.
'classified' can be an answer for 'categorised' (classifying is a kind of categorizing). I'm not certain of the 'group' bit.
'on condition that one is a journalist' is the wordplay.
'on' is an insertion indicator (as in clothing 'on' a person).
'condition' becomes 'if' (as in 'a big if').
'that one' becomes 'i' (Roman numeral).
'is a journalist' becomes 'classed' (I can't justify this - if you can you should give a lot more credence to this answer).
'if'+'i'='ifi'
'ifi' put into 'classed' is 'CLASSIFIED'.
Can you help me to learn more?
(Other definitions for classified that I've seen before include "Categorised; secret" , "to be kept secret" , "Arranged in categories" , "Ordered" .)