Writing on page about excuse (7)
I believe the answer is:
pretext
'excuse' is the definition.
(a pretext is an excuse or pretence)
'writing on page about' is the wordplay.
'writing' becomes 'text' (text is a kind of writing).
'on' is a charade indicator (letters next to each other).
'page about' becomes 'pre' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'text' put after 'pre' is 'PRETEXT'.
(Other definitions for pretext that I've seen before include "Excuse for an action" , "Justification, excuse" , "False excuse" , "Fictitious reason in order to conceal the real one" , "Ostensible justification" .)