Pursuer in search at sea (6)
I believe the answer is:
chaser
'pursuer' is the definition.
('chaser' can be a synonym of 'pursuer')
'search at sea' is the wordplay.
'at sea' is an anagram indicator.
'search' with letters rearranged gives 'CHASER'.
'in' acts as a link.
(Other definitions for chaser that I've seen before include "Horse; drink following another" , "one in the national hunt" , "Drink taken after another" , "different drink to follow" , "Drink of beer after spirit" .)