Harmony of form, a sort of m-mystery! (8)
I believe the answer is:
symmetry
'harmony of form' is the definition.
'a sort of m-mystery' is the wordplay.
'a sort of' indicates an anagram.
'm-mystery' becomes 'mmystery' ('mystery' said with a stutter like 'mystery' was).
'mmystery' anagrammed gives 'SYMMETRY'.
(Other definitions for symmetry that I've seen before include "Balance between two parts" , "Mirrored disposition about axis" , "Arrangement of parts in pleasing proportions" , "Balance in appearance" , "Pleasing proportion of the parts of a thing" .)