I'm done! Sin horrendous in magnitude! (9)
I believe the answer is:
dimension
'magnitude' is the definition.
(dimension is a kind of magnitude)
'i'm done sin horrendous' is the wordplay.
'horrendous' is an anagram indicator (I've seen this in other clues).
'im'+'done'+'sin'='imdonesin'
'imdonesin' is an anagram of 'DIMENSION'.
'in' is the link.
(Other definitions for dimension that I've seen before include "size" , "Length or breadth, say" , "Proportion" , "measure" .)