It is one to join together a statement (13)
I believe the answer is:
pronouncement
'statement' is the definition.
(I know that pronouncement is a type of declaration)
'it is one' is the wordplay.
'it' becomes 'pronoun' ('it' is a pronoun).
'is one' becomes 'cement' (I am not sure about this - if you are sure you should believe this answer much more).
'pronoun'+'cement'='PRONOUNCEMENT'
'to join together a' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pronouncement that I've seen before include "Edict" , "official statement" , "An authoritative declaration" , "ordinance" , "Official declaration" .)