Fish both ways in the sea (6)
I believe the answer is:
marlin
'fish' is the definition.
(type of fish)
'both ways in the sea' is the wordplay.
'both ways' becomes 'RL' (abbreviations for right and left).
'in' indicates putting letters inside.
'the sea' becomes 'main' (synonyms).
'rl' placed into 'main' is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Large game fish" , "Spearfish" , "Sea roamer" , "type of fish" , "Large sea-fish" .)