What does the cryptic crossword clue "Atmosphere in heart of mine became explosive (8)" mean?

The definition is "Atmosphere". The wordplay uses an anagram. anagram of IN + BECAME = AMBIENCE; definition: "Atmosphere" The answer is AMBIENCE.

Atmosphere in heart of mine became explosive (8)

Guardian Cryptic #28300 | 25 Across | 2020-11-25 flower