Mild movement of sea at end of day (4)
I believe the answer is:
easy
'mild' is the definition.
(thesaurus)
'movement of sea at end of day' is the wordplay.
'movement of' indicates an anagram (I've seen 'movement' mean this).
'at end of' says to take the final letters.
The final letter of 'day' is 'y'.
'sea' with letters rearranged gives 'eas'.
'eas'+'y'='EASY'
(Other definitions for easy that I've seen before include "without difficulty" , "Gentle - light" , "Effortless, uncomplicated" , "for a piece of cake" , "Simple or uncomplicated" .)