Grime can't replace weight (9)
I believe the answer is:
centigram
'weight' is the definition.
(I've seen this before)
'grime can't replace' is the wordplay.
'replace' is an anagram indicator (I've seen 'replacing' mean this (placing again)).
'grime'+'cant'='grimecant'
'grimecant' with letters rearranged gives 'CENTIGRAM'.
(Other definitions for centigram that I've seen before include "Small mass" , "small weight" .)