What does the cryptic crossword clue "Head driller and surrogate parent (6)" mean?

The definition is "Head driller". The wordplay uses an anagram. anagram of PARENT = TREPAN; definition: "Head driller" The answer is TREPAN.

Head driller and surrogate parent (6)

Guardian Cryptic #29125 | 19 Across | 2023-07-18 flower