The most common and most likely answer to this crossword clue is the 13 letter word SEDENTARINESS
S
E
D
E
N
T
A
R
I
N
E
S
S
More clues leading to the result SEDENTARINESS
Buddy explains!
The straight part
We know that "Tending to sit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tending to sit = sedentariness.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.