The most common and most likely answer to this crossword clue is the 10 letter word GRANDDADDY
G
R
A
N
D
D
A
D
D
Y
More clues leading to the result GRANDDADDY
Buddy explains!
The straight part
We know that "Oldster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oldster = granddaddy.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 GRANDDADDY mean?
GRANDDADDY noun- A grandfather.
- Something that is the greatest or most significant of its kind.