Declare old application (7)
I believe the answer is:
exclaim
'declare' is the definition.
(I know that declare is a more specific form of exclaim)
'old application' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'application' becomes 'claim'.
'ex'+'claim'='EXCLAIM'
(Other definitions for exclaim that I've seen before include "Cry out suddenly, as from pain or surprise" , "Blurt out" , "Speak sharply" , "Cry out (in shock, etc)" , "Declare" .)