Diets affected by sea changes (5)
I believe the answer is:
tides
'sea changes' is the definition.
'diets affected' is the wordplay.
'affected' indicates an anagram.
'diets' anagrammed gives 'TIDES'.
'by' is the link.
(Other definitions for tides that I've seen before include "Lunar-driven sea movements (5)" , "Periodic ebbs and flows of the sea" , "Sited (anag.)" , "Lunar ocean movements" , "They ebb and flow" .)