As it grows up it grows down (8)
I believe the answer is:
duckling
'it grows up it grows down' is the definition.
The definition suggests a singular noun which matches the answer.
'as' is the wordplay.
I cannot really see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for duckling that I've seen before include "Young swimming bird" , "little fowl" , "Young waterfowl" , "Young mallard, for instance" , "Young swimmer" .)