Turtle with purple head lost ground (7)
I believe the answer is:
terrain
'ground' is the definition.
(terrain is land or ground)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for terrain that I've seen before include "(Type of) ground" , "Type of countryside" , "Tract of land of a specific character" , "A stretch of land for trainer" , "Tract of land with particular character" .)