Figure is round, not entirely lifeless (6)
I believe the answer is:
isogon
'figure' is the definition.
(I have seen 'Regular figure' mean 'isogon' so perhaps 'figure' could also mean 'isogon')
'is round not entirely lifeless' is the wordplay.
'round' says the letters should be written in reverse.
'not' becomes 'no' (negative).
'entirely lifeless' becomes 'go' (I can't justify this - if you can you should believe this answer much more).
'no'+'go'='nogo'
'nogo' in reverse letter order is 'ogon'.
'is'+'ogon'='ISOGON'
Can you help me to learn more?
(Other definitions for isogon that I've seen before include "Regular figure" , "Equiangular polygon" , "See plane figure" , "Equilateral triangle, say" .)