Regular shape in keeping very fit (6)
I believe the answer is:
'regular shape' is the definition.
(isogon is a type of regular shape)
'in keeping very fit' is the wordplay.
'keeping' is an insertion indicator.
'very' becomes 'so' (both can mean 'extremely').
'fit' becomes 'go' (go can mean to fit or match).
'in' enclosing 'sogo' is 'ISOGON'.
(Other definitions for isogon that I've seen before include "Equilateral triangle, say" , "See plane figure" , "Regular figure" , "Equiangular polygon" .)