Smooth-shelled nut — tropical waterfowl dropping large one (5)
I believe the answer is:
pecan
'smooth-shelled nut tropical waterfowl dropping' is the definition.
I can't judge whether this definition defines the answer.
'large one' is the wordplay.
'large' becomes 'pec' (I can't explain this - if you can you should believe this answer much more).
'one' becomes 'an' (an apple is one apple).
'pec'+'an'='PECAN'
Can you help me to learn more?
(Other definitions for pecan that I've seen before include "Type of nut" , "Walnut-like nut" , "US nut tree" , "Something like a walnut" , "Species of hickory - edible nut" .)