Island business captures old carbon in geological formation (8)
I believe the answer is:
isocline
'geological formation' is the definition.
(type of rock layer)
'island business captures old carbon' is the wordplay.
'island' becomes 'is' (abbreviation).
'business' becomes 'line' (as in someone's line of work).
'captures' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'carbon' becomes 'C' (chemical symbol).
'is'+'line'='isline'
'o'+'c'='oc'
'isline' placed around 'oc' is 'ISOCLINE'.
'in' acts as a link.
(Other definitions for isocline that I've seen before include "Rock form" , "Geological feature that is folded" , "Area of folded rock strata" .)