Animal quietly entering countryside (8)
I believe the answer is:
terrapin
'animal' is the definition.
The answer is an example.
'quietly entering countryside' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'entering' means one lot of letters goes inside another.
'countryside' becomes 'terrain' (terrain means land).
'p' put inside 'terrain' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Web-footed turtle" , "edible, freshwater reptile" , "Partner I found for freshwater tortoise" , "Swimmer" , "Small freshwater turtle" .)