The most common and most likely answer to this crossword clue is the 11 letter word EVAPORATION
I've seen this crossword clue in The Guardian
E
V
A
P
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Buddy explains!
The straight part
We know that "Change of state" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case change of state = evaporation.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 EVAPORATION mean?
EVAPORATION noun- The process of a liquid converting to the gaseous state.
- The process in which all or a portion of liquid (in a container) is turned into vapour, in order to increase the concentration of solid matter in the mixture.
- That which is evaporated; vapor.