... the dictionary containing proclamation (5)
I believe the answer is:
edict
'proclamation' is the definition.
(I know that edict is a type of proclamation)
'the dictionary containing' is the wordplay.
'containing' says the answer is hidden in the clue (I've seen 'contains' mean this).
'EDICT' can be found hidden inside 'the dictionary'.
(Other definitions for edict that I've seen before include "Cited an official proclamation" , "Formal order" , "Legally binding command" , "An official command" , "Order issued by an authority" .)