Extreme and irrational fear (6)
I believe the answer is:
phobia
'extreme and irrational fear' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for phobia that I've seen before include "Extreme and irrational fear" , "Great fear of something" , "Morbid, irrational fear" , "Obsessive and persistent fear of something" , "Irrational hatred" .)