Ailment resulting from seaside upset (7)
I believe the answer is:
disease
'ailment' is the definition.
(I know that ailment can be written as disease)
'seaside upset' is the wordplay.
'upset' indicates an anagram.
'seaside' with letters rearranged gives 'DISEASE'.
'resulting from' is the link.
(Other definitions for disease that I've seen before include "Ailment, malady" , "blight" , "Bodily disorder" , "Seaside (anag) -- ailment" , "affection" .)