Fish right and left in the open sea (6)
I believe the answer is:
marlin
'fish' is the definition.
(type of fish)
'right and left in the open sea' is the wordplay.
'right' becomes 'r' (common abbreviation).
'and' means one lot of letters go next to another.
'left' becomes 'L' (common abbreviation).
'in' indicates putting letters inside.
'the open sea' becomes 'main' (the main is an old name for the sea).
'r'+'l'='rl'
'rl' put inside 'main' is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Prized game fish" , "Large oceanic sport fish" , "Spearfish" , "Sea roamer" , "type of fish" .)