What does the cryptic crossword clue "Old Belloc novel, one of many in circulation (5,4)" mean?

The wordplay uses an anagram. anagram of OLD + BELLOC = BLOOD CELL The answer is BLOOD CELL.

Old Belloc novel, one of many in circulation (5,4)

Times Cryptic #28746 | 6 Down | 2023-10-28 flower