Blood line passing through Dad’s mother (6)
I believe the answer is:
plasma
'blood' is the definition.
(thesaurus)
'line passing through dad's mother' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'passing through' is an insertion indicator.
'dad's' becomes 'pa's' ('pa' can be a synonym of 'dad').
'mother' becomes 'ma' (ma can mean mother).
'l' placed into 'pas' is 'plas'.
'plas'+'ma'='PLASMA'
(Other definitions for plasma that I've seen before include "hot stuff" , "Star material" , "Part of blood; ionised material" , "Blood component and type of television" , "A psalm (anag.)" .)