Seaside treatment for complaint (7)
I believe the answer is:
disease
'complaint' is the definition.
(I've seen this before)
'seaside treatment' is the wordplay.
'treatment' is an anagram indicator.
'seaside' anagrammed gives 'DISEASE'.
'for' acts as a link.
(Other definitions for disease that I've seen before include "Seaside (anag) -- ailment" , "scrapie for example" , "Contagion" , "scurvy?" , "blight" .)