Malcolm's fat water bird (7)
I believe the answer is:
mallard
'water bird' is the definition.
(I know that mallard is a type of duck)
'malcolm's fat' is the wordplay.
'malcolm' becomes 'mal' (I am not sure about this - if you are sure you should believe this answer much more).
'fat' becomes 'lard' (lard is a kind of fat).
'mal'+'lard'='MALLARD'
Can you help me to learn more?
(Other definitions for mallard that I've seen before include "The famous steam train" , "Engine holding the world speed record for a steam locomotive, set at 123mph in 1938" , "Type of duck - world speed record holding steam engine (1938)" , "Kind of duck" , "web-footed bird" .)