Abnormal fear (6)
I believe the answer is:
phobia
'abnormal fear' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for phobia that I've seen before include "Irrational and lasting fear" , "An irrational fear or dislike" , "Irrational hatred" , "Aversion" , "A lasting abnormal fear of something" .)