Strange organisation is unaffected (9)
I believe the answer is:
unnatural
'strange' is the definition.
(I've seen this before)
'organisation is unaffected' is the wordplay.
'organisation' becomes 'un' (UN is an example).
'is unaffected' becomes 'natural' ('natural' can be a synonym of 'unaffected').
'un'+'natural'='UNNATURAL'
(Other definitions for unnatural that I've seen before include "Freakish" , "Affected" , "Distorted" , "Artificial" , "Perverse" .)