What does the cryptic crossword clue "Modern sound then obvious, basic to the core? (7)" mean?

The wordplay uses a homophone (sounds like). NU [sounds like ‘new’ – modern] + CLEAR [obvious] The answer is NUCLEAR.

Modern sound then obvious, basic to the core? (7)

Guardian Cryptic #26511 | 10 Across | 2015-03-05 flower