Attempt to cover one diary in three books (7)
I believe the answer is:
trilogy
'three books' is the definition.
(set of three books)
'attempt to cover one diary' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'cover' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'diary' becomes 'log' (log can mean a record or diary).
'i'+'log'='ilog'
'try' placed around 'ilog' is 'TRILOGY'.
'in' is the link.
(Other definitions for trilogy that I've seen before include "Group of three related works" , "series" , "Group of three novels, films, etc" , "Three-volume work" , "Work in three parts" .)