Letter from Greek grips old boy with a fear (6)
I believe the answer is:
phobia
'fear' is the definition.
(a phobia is a fear of a particular thing)
'letter from greek grips old boy with a' is the wordplay.
'letter from greek' becomes 'phi' (letter from the Greek alphabet).
'grips' is an insertion indicator.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' means one lot of letters go next to another.
'phi' placed around 'ob' is 'phobi'.
'phobi'+'a'='PHOBIA'
(Other definitions for phobia that I've seen before include "Irrational fear (of thing)" , "Aversion" , "Irrational and lasting fear" , "Abnormal lasting fear or anxiety" , "A lasting abnormal fear or dislike" .)