Fabulous creature found in deeper oceans (3)
I believe the answer is:
roc
'fabulous creature' is the definition.
(I know that roc is a type of legendary bird)
'in deeper oceans' is the wordplay.
'in' says the answer is hidden in the clue.
'ROC' is hidden within 'deeper oceans'.
'found' acts as a link.
(Other definitions for roc that I've seen before include "Legendary thing" , "Bird imagined" , "Mythical creature from Arabia" , "Vast bird of legend" , "fabulous bird is seen" .)