Flat broad limb of aquatic animals (7)
I believe the answer is:
flipper
'flat broad limb of aquatic animals' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for flipper that I've seen before include "Limb used for swimming" , "Flat broad limb of aquatic animals" , "feature of dolphin?" , "Shoe for swimming" , "Seal/turtle limb" .)