Write on this — not extended record, short announcement (7)
I believe the answer is:
notepad
'write on this' is the definition.
(a notepad can be writtenon)
'not extended record short announcement' is the wordplay.
'extended record' becomes 'EP' (abbreviation).
'short announcement' becomes 'ad' (short for advert; 'short' indicates an abbreviation).
'not'+'ep'+'ad'='NOTEPAD'
(Other definitions for notepad that I've seen before include "Block writer might have" , "writer's block" , "Pocket-sized computer" , "Item to write on" , "type of computer" .)