A large number study in fear (5)
I believe the answer is:
dread
'fear' is the definition.
(dread is a kind of fear)
'a large number study' is the wordplay.
'a large number' becomes 'd' (five hundred in Roman numerals).
'study' becomes 'read' (synonyms).
'd'+'read'='DREAD'
'in' acts as a link.
(Other definitions for dread that I've seen before include "Frightful" , "Dared to fear greatly" , "Fear - dire" , "quail" , "Adder (anag)" .)