What does the cryptic crossword clue "Parthian peer replaced as one expected to succeed (4,8)" mean?

The wordplay uses an anagram. anagram of PARTHIAN + PEER = HEIRAPPARENT The answer is HEIRAPPARENT.

Parthian peer replaced as one expected to succeed (4,8)

Guardian Cryptic #23527 | 12 Across | 2005-08-08 flower