Name in diary as of old (4,5)
I believe the answer is:
long since
'of old' is the definition.
(thesaurus)
'name in diary as' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'in' indicates putting letters inside.
'diary' becomes 'log' (log can mean a record or diary).
'as' becomes 'since' (both can mean 'because').
'n' placed within 'log' is 'long'.
'long'+'since'='LONG SINCE'
(Other definitions for long since that I've seen before include "for many years" , "in the distant past" , "A good time ago" .)