Reptile quietly buried in ground (8)
I believe the answer is:
'reptile' is the definition.
(terrapin is a kind of reptile)
'quietly buried in ground' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'buried in' means one lot of letters goes inside another.
'ground' becomes 'terrain' (I've seen this before).
'p' going within 'terrain' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Water tortoise" , "Small species of turtle" , "Give out" , "freshwater creature" , "Aquatic tortoise" .)