Dull parent replaces contents of diary (6)
I believe the answer is:
dreary
'dull' is the definition.
(similar in meaning)
'parent replaces contents of diary' is the wordplay.
I cannot quite see how this works, but
'parent' could be 'rear' (synonyms) and 'rear' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for dreary that I've seen before include "Cheerless and dull" , "Gloomy - dull" , "Dismal, funereal" , "Drab and dismal" , "Cheerless and depressing" .)