Woman in yacht at sea (7)
I believe the answer is:
cynthia
'woman' is the definition.
'in yacht at sea' is the wordplay.
'at sea' is an anagram indicator.
'in'+'yacht'='inyacht'
'inyacht' with letters rearranged gives 'CYNTHIA'.
(Other definitions for cynthia that I've seen before include "Girl's name" , "Girl" , "Woman" .)