Irrational fear (6)
I believe the answer is:
phobia
'irrational fear' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for phobia that I've seen before include "Pathological fear" , "Obsessive and persistent fear of something" , "Great fear of something" , "A lasting abnormal fear of something" , "Aversion" .)