Relative of tortoise quietly lost ground (7)
I believe the answer is:
terrain
'ground' is the definition.
(I've seen this before)
'relative of tortoise quietly lost' is the wordplay.
'relative of tortoise' becomes 'terrapin' (type of turtle).
'quietly' becomes 'p' (p for piano and quiet).
'lost' suggests deleting specific letters.
'terrapin' with 'p' taken out is 'TERRAIN'.
(Other definitions for terrain that I've seen before include "Land of a particular type" , "Tract of land with prevailing characteristic" , "Tract of land of a specific character" , "(Type of) ground" , "Topography" .)