Odd seaside complaint (7)
I believe the answer is:
disease
'complaint' is the definition.
(I've seen this before)
'odd seaside' is the wordplay.
'odd' indicates an anagram.
'seaside' anagrammed gives 'DISEASE'.
(Other definitions for disease that I've seen before include "affection" , "Bodily disorder" , "Seaside (anag) -- ailment" , "scrapie for example" , "Seaside (anag) - complaint" .)