Di not Jo's written in diary every day (7)
I believe the answer is:
diurnal
'every day' is the definition.
(I've seen this before)
'di not' is the wordplay.
'not' becomes 'urnal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'di'+'urnal'='DIURNAL'
'jo's written in diary' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for diurnal that I've seen before include "in daylight" , "I run lad in daily mode, not nocturnal" , "every day" , "Occurring daily" , "Active during the day" .)