Facility functions in rough sea (8)
I believe the answer is:
easiness
'facility' is the definition.
(I've seen this in another clue)
'functions in rough sea' is the wordplay.
'functions' becomes 'sines' (sine is a kind of function).
'in' is an insertion indicator.
'rough' indicates an anagram (a rough or broken ordering of the letters).
'sea' with letters rearranged gives 'eas'.
'sines' placed into 'eas' is 'EASINESS'.
(Other definitions for easiness that I've seen before include "Lack of difficulty" , "Nonchalance" , "Facile quality" .)