From son to father (6)
I believe the answer is:
'from son to father' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for filial that I've seen before include "Appropriate to issue" , "Of or due from son or daughter, perhaps affection" , "Dutiful like son or daughter" , "Relating to sons or daughters" , "Proper to a son or daughter" .)