Marge spread round top of muffin, a very small amount (6)
I believe the answer is:
gramme
'a very small amount' is the definition.
(small weight)
'marge spread round top of muffin' is the wordplay.
'spread' indicates an anagram.
'round' is an insertion indicator.
'top of' indicates taking the first letters.
The initial letter of 'muffin' is 'm'.
'marge' with letters rearranged gives 'grame'.
'grame' placed around 'm' is 'GRAMME'.
(Other definitions for gramme that I've seen before include "Small unit of mass" , "Arm Meg (anag)" , "Metric unit of weight" , "One thousandth of a kilogram (Fr)" , "Small weight" .)