Bit of water required by robust aquatic mammal (5)
I believe the answer is:
whale
'aquatic mammal' is the definition.
(whale is a kind of aquatic mammal)
'bit of water required by robust' is the wordplay.
'bit of' indicates taking the first letters.
'required by' is a charade indicator (letters next to each other).
'robust' becomes 'hale' (in good health).
The initial letter of 'water' is 'w'.
'w'+'hale'='WHALE'
(Other definitions for whale that I've seen before include "One wallowing in the drink" , "Patch of oil, on water" , "Large animal" , "Leviathan" , "Moby-Dick" .)