The most common and most likely answer to this crossword clue is the 3 plus 3 plus 3 letter word THE OLD MAN
I've seen this crossword clue in The Guardian
T
H
E
O
L
D
M
A
N
Buddy explains!
The straight part
We know that "Father" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case father = the old man.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.