The most common and most likely answer to this crossword clue is the 3 plus 4 letter word OLD FART
I've seen this crossword clue in The Guardian
O
L
D
F
A
R
T
More clues leading to the result OLD FART
Buddy explains!
The straight part
We know that "They’ve seen it all" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’ve seen it all = old fart.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.