Subdued, thanks to sea (5)
I believe the answer is:
tamed
'subdued' is the definition.
('tame' can be a synonym of 'subdue')
'thanks to sea' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to sea' becomes 'med' (abbreviation of Mediterranean**).
'ta'+'med'='TAMED'
(Other definitions for tamed that I've seen before include "Of animals, domesticated" , "Reined" , "now behaving himself?" , "Domesticated or subdued" .)