Shrewd expression of disapproval, entering turbulent sea (6)
I believe the answer is:
astute
'shrewd' is the definition.
('astute' can be a synonym of 'shrewd')
'expression of disapproval entering turbulent sea' is the wordplay.
'expression of disapproval' becomes 'tut' (I've seen this before).
'entering' means one lot of letters goes inside another.
'turbulent' is an anagram indicator.
'sea' with letters rearranged gives 'ase'.
'tut' put into 'ase' is 'ASTUTE'.
(Other definitions for astute that I've seen before include "Discerning" , "Perspicacious" , "Sharp-witted" , "Able to assess (people or situations) accurately" , "Wide" .)