Are they silly enough to get fatally fat? (5)
I believe the answer is:
geese
'are they' is the definition.
The definition suggests a plural noun which matches the answer.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for geese that I've seen before include "Large farm birds" , "Large water-birds with webbed feet and broad bills" , "Protectors of city" , "Birds that fly in a skein" , "They come in gaggles and skeins" .)