It's rocky as result of drop in share (8)
I believe the answer is:
dolomite
'it's rocky' is the definition.
(type of rock)
'drop in share' is the wordplay.
'drop' becomes 'omit' (synonyms).
'in' is an insertion indicator.
'share' becomes 'dole' (dole is a kind of share).
'omit' put within 'dole' is 'DOLOMITE'.
'as result of' acts as a link.
(Other definitions for dolomite that I've seen before include "stone" , "Limestone rock" , "Form of rock" , "Mineral after which a range of Italian Alps is named" .)