Struck by a most unusual eye-opener (5)
I believe the answer is:
smote
'struck' is the definition.
(I've seen this before)
'a most unusual eye-opener' is the wordplay.
'unusual' indicates an anagram.
'opener' suggests taking the first letters.
The initial letter of 'eye' is 'e'.
'most' anagrammed gives 'smot'.
'smot'+'e'='SMOTE'
'by' is the link.
(Other definitions for smote that I've seen before include "Hit" , "Struck firmly" .)