Small amount of marble initially included in exotic design (7)
I believe the answer is:
smidgen
'small amount' is the definition.
'marble initially included in exotic design' is the wordplay.
'initially' says to take the initial letters.
'included in' means one lot of letters goes inside another.
'exotic' is an anagram indicator.
The first letter of 'marble' is 'm'.
'design' is an anagram of 'sidgen'.
'm' going within 'sidgen' is 'SMIDGEN'.
'of' acts as a link.
(Other definitions for smidgen that I've seen before include "A very small amount (7)." , "bit!" , "A tad" , "Tiny amount" , "Very little" .)