The most common and most likely answer to this crossword clue is the 15 letter word ACCLIMATISATION
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "getting used to things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case getting used to things = acclimatisation.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.
Crossword Answer definitions
ACCLIMATISATION noun- The act of acclimatizing; the process of inuring to a new climate, or the state of being so inured.
- The climatic adaptation of an organism that has been moved to a new environment.