Ruddy duck? That's a wader! (8)
I believe the answer is:
flamingo
'that's a wader' is the definition.
(flamingo is a kind of wader)
'ruddy duck?' is the wordplay.
'ruddy' becomes 'flaming' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'duck?' becomes 'o' (resembles zero - 'duck' in cricket).
'flaming'+'o'='FLAMINGO'
Can you help me to learn more?
(Other definitions for flamingo that I've seen before include "Tall wading bird" , "Long-legged wader" , "Pretty bird" , "Exotic bird" , "Pink wader" .)