Fear of onset of darkness? Study needed (5)
I believe the answer is:
dread
'fear' is the definition.
('dread' can be a synonym of 'fear')
'onset of darkness? study needed' is the wordplay.
'onset of' says to take the initial letters.
'study' becomes 'read' ('read' can be a synonym of 'study').
'needed' is a charade indicator (letters next to each other).
The first letter of 'darkness' is 'd'.
'd'+'read'='DREAD'
'of' acts as a link.
(Other definitions for dread that I've seen before include "Disquiet" , "Be scared of" , "Apprehensive fear" , "Affright" , "Great fear or apprehension" .)