A wrong guess about customs (6)
I believe the answer is:
usages
'customs' is the definition.
(I know this)
'a wrong guess about' is the wordplay.
'a wrong' indicates an anagram.
'about' becomes 'a' (abbreviation in Chambers).
'guess'+'a'='guessa'
'guessa' with letters rearranged gives 'USAGES'.
(Other definitions for usages that I've seen before include "Customary treatments, practices" , "Habitual practices with words" , "Habitual or customary practices" , "Linguistic norms" , "Customs." .)