Windy hollow around middle of ground (7)
I believe the answer is:
sinuous
'windy' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'hollow around middle of ground' is the wordplay.
'hollow' becomes 'sinus' (I've seen this before).
'around' means one lot of letters goes inside another.
'middle of' means to look at the middle letters.
The centre of 'ground' is 'ou'.
'sinus' enclosing 'ou' is 'SINUOUS'.
Can you help me to learn more?
(Other definitions for sinuous that I've seen before include "Full of curves, bends" , "Having many curves and turns" , "Wiggly" , "Curving in and out" , "Twisting and turning" .)