Man being one is extremely lithe (4)
I believe the answer is:
isle
'man' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for isle that I've seen before include "Small area of ground" , "one ringed in the main?" , "Land amid water" , "eg Hoy, Yell or Sark" , "Dogs, say" .)