Ailment resulting from seaside upset (7)
I believe the answer is:
disease
'ailment' is the definition.
(I've seen this in another clue)
'seaside upset' is the wordplay.
'upset' indicates an anagram.
'seaside' is an anagram of 'DISEASE'.
'resulting from' acts as a link.
(Other definitions for disease that I've seen before include "Ailment, malady" , "blight" , "Bodily disorder" , "Seaside (anag) -- ailment" , "affection" .)