One swims in under rock (6)
I believe the answer is:
marlin
'one swims' is the definition.
(type of fish)
'in under rock' is the wordplay.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'rock' becomes 'marl' (I've seen this before).
'in' after 'marl' is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Prized game fish" , "Large ocean fish" , "Large sea-fish" , "Large oceanic sport fish" , "type of fish" .)