Irrational aversion (6)
I believe the answer is:
phobia
'irrational aversion' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for phobia that I've seen before include "Abnormal lasting fear or anxiety" , "Great fear of something" , "Irrational and lasting fear" , "A lasting abnormal fear of smoething" , "A lasting abnormal fear of something" .)