Unusual seaside shingles for instance (7)
I believe the answer is:
disease
'shingles for instance' is the definition.
(I know that herpes zoster is a type of herpes)
'unusual seaside' is the wordplay.
'unusual' indicates an anagram.
'seaside' with letters rearranged gives 'DISEASE'.
(Other definitions for disease that I've seen before include "Impairment of health" , "scrapie for example" , "Illness, malady" , "blight" , "Contagion" .)