Choppy sea, ultimately stormy, not difficult (4)
I believe the answer is:
easy
'not difficult' is the definition.
('easy' is the opposite to 'difficult')
'choppy sea ultimately stormy' is the wordplay.
'choppy' indicates an anagram.
'ultimately' suggests the final letters.
The final letter of 'stormy' is 'y'.
'sea' is an anagram of 'eas'.
'eas'+'y'='EASY'
(Other definitions for easy that I've seen before include "Achieved without effort" , "Simply achieved" , "Tranquil" , "Child's play" , "Convenient" .)