Present tense used in log that's passed down (10)
I believe the answer is:
hereditary
'passed down' is the definition.
(passed down genetically)
'present tense used in log' is the wordplay.
'present' becomes 'here' (both can mean in attendance).
'tense' becomes 't' (abbreviation used in many dictionaries).
'used in' is an insertion indicator.
'log' becomes 'diary' (I've seen this before).
't' put inside 'diary' is 'ditary'.
'here'+'ditary'='HEREDITARY'
'that's' acts as a link.
(Other definitions for hereditary that I've seen before include "Passed on by parents" , "Transmitted" , "Passed on by genetic factors" , "From ancestors" , "Family" .)