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' (synonyms).
'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" .)