The most common and most likely answer to this crossword clue is the 16 letter word SHRINK-RESISTANT
I've seen this crossword clue in The Guardian
S
H
R
I
N
K
-
R
E
S
I
S
T
A
N
T
Buddy explains!
The straight part
We know that "In terms of habits, not getting any less" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in terms of habits, not getting any less = shrink-resistant.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.