Successful ditchers preparing big cold flush (8)
I believe the answer is:
goldfish
'flush' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'successful ditchers preparing big cold' is the wordplay.
I cannot really see how this works, but
'cold' could be 'old' (synonyms) and 'old' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for goldfish that I've seen before include "Place lacking privacy" , "Small orange carp" , "Popular piscine pet" , "Domestic pet with an approximate 11-second memory" , "one enjoying pond life?" .)