Large fish right and left in sea (6)
I believe the answer is:
marlin
'large fish' is the definition.
(I've seen this before)
'right and left in sea' is the wordplay.
'right' becomes 'r' (common abbreviation).
'and' says to put letters next to each other.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'sea' becomes 'main' (synonyms).
'r'+'l'='rl'
'rl' placed inside 'main' is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Large sea-fish" , "Spearfish" , "type of fish" , "Large ocean fish" , "Sea roamer" .)