Quack, quack! Duck walks in at the start (10)

'quack quack duck walks' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.

'in at the start' is the wordplay.
I cannot really see how this works, but
'in' could be 'home' ('he's in' can mean 'he's home') and 'home' is found within the answer.
'at' is found in the answer.
This explanation may well be incorrect...