Don's first to study alarm (5)
I believe the answer is:
dread
'alarm' is the definition.
(I've seen this before)
'don's first to study' is the wordplay.
'don's first' becomes 'd' (1st letter of 'don').
'to study' becomes 'read' ('read' can be a synonym of 'study').
'd'+'read'='DREAD'
(Other definitions for dread that I've seen before include "Be very averse to" , "Anticipate with horror" , "Awe" , "Dared to fear greatly" , "Disquiet" .)