The most common and most likely answer to this crossword clue is the 3 plus 12 letter word AIR CONDITIONING
I've seen this crossword clue in The Guardian
A
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More clues leading to the result AIR CONDITIONING
Buddy explains!
The straight part
We know that "way of regulating the environment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case way of regulating the environment = air conditioning.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 AIR CONDITIONING mean?
AIR CONDITIONING noun- The state of temperature and humidity produced by an air conditioner.
- An air conditioner or system of air conditioners.
- To mechanically cool a space or an entire structure.