One trial involving a member of the family (8)
I believe the answer is:
relation
'a member of the family' is the definition.
(I've seen this before)
'one trial involving' is the wordplay.
'involving' is an anagram indicator.
'one'+'trial'='onetrial'
'onetrial' anagrammed gives 'RELATION'.
(Other definitions for relation that I've seen before include "Oriental kind of family connection" , "Connection; narrative" , "wife?" , "Kinsman; narrative" , "Account" .)