Agin sex law being amended: it used to be applied to the letter (7,3)
I believe the answer is:
sealing wax
'it used to be applied to the letter' is the definition.
The definition suggests a singular noun which matches the answer.
'agin sex law being amended' is the wordplay.
'being amended' indicates an anagram.
'agin'+'sex'+'law'='aginsexlaw'
'aginsexlaw' is an anagram of 'SEALING-WAX'.
Can you help me to learn more?
(Other definitions for sealing wax that I've seen before include "Resinous compound used on formal documents" , "It is used for closing letters, documents etc" , "resinous substance" , "I used to stamp letter" , "It is heated and dripped onto documents" .)