Not too difficult a puzzle, I thought. Solving time, 15 mins. All the four 15-letter entries were direct anagrams.
Notes on some clues below – happy to explain others if asked.
ACROSS
1 A RAM IS One of the Three Musketeers
17 T(he) ORC
DOWN
5 CA(u)SE heart = central letter. Definition: odd character
9 A ITCH The letter (character) H
15 S (AMOS) A 2-letter crossword staple SA (sex appeal) = IT
18 CORPS “Core” i.e. not peripheral