Fish in sheltered marine area turning up (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a kind of fish)
'sheltered marine area turning up' is the wordplay.
'sheltered marine area' becomes 'lee' (the side of a ship sheltered from wind).
'turning up' is a reversal indicator.
'lee' reversed gives 'EEL'.
'in' is the link.
(Other definitions for eel that I've seen before include "Almost scaleless fish" , "Slender fish" , "Sea creature like conger" , "Moray or electric, for instance" , "Slippery creature" .)