The most common and most likely answer to this crossword clue is the 12 letter word SEXAGENARIAN
S
E
X
A
G
E
N
A
R
I
A
N
Buddy explains!
The straight part
We know that "senior citizen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case senior citizen = sexagenarian.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 SEXAGENARIAN mean?
SEXAGENARIAN noun- A person who is 60 years old or between the ages of 60 and 69.
- Being between the age of 60 and 69, inclusive. In one's seventh decade.