Entering a quiet sea, one's sheepish (7)
I believe the answer is:
ashamed
'sheepish' is the definition.
(similar in meaning)
'entering a quiet sea one's' is the wordplay.
'entering' indicates putting letters inside.
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'sea' becomes 'med' (abbreviation of Mediterranean).
'one' becomes 'a' (a thing is one thing).
'a'+'sh'+'med'='ashmed'
'ashmed' going around 'a' is 'ASHAMED'.
(Other definitions for ashamed that I've seen before include "Feeling remorse and guilt" , "Troubled by feelings of guilt or embarrassment" , "Discomfited" , "hanging his head" , "Having painful regret for behaviour" .)