One's shelled quietly coming into land (8)
I believe the answer is:
terrapin
'one's shelled' is the definition.
I can't judge whether this defines the answer.
'quietly coming into land' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'coming into' is an insertion indicator.
'land' becomes 'terrain' (terrain is land or territory).
'p' inserted into 'terrain' is 'TERRAPIN'.
Can you help me to learn more?
(Other definitions for terrapin that I've seen before include "Partner I found for freshwater tortoise" , "Small freshwater turtle" , "Little turtle" , "Hardback" , "Swimmer" .)