The most common and most likely answer to this crossword clue is the 8 letter word GRANDDAD
I've seen this crossword clue in The Times
G
R
A
N
D
D
A
D
More clues leading to the result GRANDDAD
Buddy explains!

The straight part
We know that "older relative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case older relative = granddad.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.
What does GRANDDAD mean?
GRANDDAD noun- Grandfather
- A familiar or disparaging term of address to an old man.