Meaning small mace isn't used (9)
I believe the answer is:
semantics
'meaning' is the definition.
(semantics is the study of meaning)
'small mace isn't used' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'used' is an anagram indicator.
'mace'+'isnt'='maceisnt'
'maceisnt' anagrammed gives 'emantics'.
's'+'emantics'='SEMANTICS'
(Other definitions for semantics that I've seen before include "Meaning to study" , "The meaning attached to words" , "Branch of linguistics to do with meaning" , "Main sects (anag.)" , "The significance of a text" .)