By way of example, they make firm declaration (13)
I believe the answer is:
pronouncement
'declaration' is the definition.
(I know that pronouncement is a type of declaration)
'by way of example they make firm' is the wordplay.
'by' says to put letters next to each other.
'way of example they' becomes 'pronoun' (I can't justify this - if you can you should believe this answer much more).
'make firm' becomes 'cement' (I've seen this in another clue).
'pronoun' next to 'cement' is 'PRONOUNCEMENT'.
Can you help me to learn more?
(Other definitions for pronouncement that I've seen before include "Edict" , "Official declaration" , "assertion" , "ordinance" , "official statement" .)