Former pupil, during hip operation, encounters a fear (6)
I believe the answer is:
phobia
'fear' is the definition.
(a phobia is a fear of a particular thing)
'former pupil during hip operation encounters a' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'during' is an insertion indicator.
'hip operation encounters' becomes 'phi' (I can't explain this - if you can you should believe this answer much more).
'ob' inserted inside 'phi' is 'phobi'.
'phobi'+'a'='PHOBIA'
Can you help me to learn more?
(Other definitions for phobia that I've seen before include "Irrational fear (of thing)" , "A lasting abnormal fear or dislike" , "Morbid, irrational fear" , "Anxiety disorder" , "Irrational and lasting fear" .)