Weight supported by robust creature (5)
I believe the answer is:
whale
'creature' is the definition.
(I have seen 'Large animal' mean 'whale' so perhaps 'animal' could also mean 'whale')
'weight supported by robust' is the wordplay.
'weight' becomes 'w' (short for weight).
'supported by' means one lot of letters go next to another (in a down clue, some letters hold up others).
'robust' becomes 'hale' (in good health).
'w'+'hale'='WHALE'
(Other definitions for whale that I've seen before include "Large cetacean" , "Great sea mammal" , "Mammal like Moby Dick" , "having such a good time?" , "Large marine mammal" .)