Kind of seaside complaint (7)
I believe the answer is:
disease
'complaint' is the definition.
(I've seen this before)
'kind of seaside' is the wordplay.
'kind of' is an anagram indicator.
'seaside' with letters rearranged gives 'DISEASE'.
(Other definitions for disease that I've seen before include "scurvy?" , "22 [MALADY]" , "Sickness" , "scrapie for example" , "affection" .)