Diary about daughter is something precious (4)
I believe the answer is:
gold
'something precious' is the definition.
I know that 'precious' relates to this answer.
'diary about daughter' is the wordplay.
'diary' becomes 'log' (log can mean a record or diary).
'about' says the letters should be written in reverse.
'daughter' becomes 'd' (genealogical abbreviation).
'log' backwards is 'gol'.
'gol'+'d'='GOLD'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for gold that I've seen before include "Element (Au)" , "Thin sheet of precious metal" , "Tokens of marriage" , "Woman after a man's money" , "gift from a king" .)