High-IQ organisation (5)
I believe the answer is:
mensa
'high-iq organisation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mensa that I've seen before include "High-IQ organisation" , "High IQ_organisation" , "Organisation for those with high IQs" , "international society" , "High-IQ society" .)