Initiate story for grandson, perhaps (5,8)
I believe the answer is:
'for grandson perhaps' is the definition.
(grandson is a type of relative)
'initiate story' is the wordplay.
'initiate' becomes 'blood' (blood can mean to initiate someone in an activity).
'story' becomes 'relation' (relation is a kind of story).
(Another definition for blood relation that I've seen is " Relative by birth".)