What does the cryptic crossword clue "Ill, Mark’s failing to meet objective (3,6)" mean?

The wordplay uses a charade (building blocks). OFF (synonym="Ill") + TARGET (synonym="Mark") = OFFTARGET The answer is OFFTARGET.

Ill, Mark’s failing to meet objective (3,6)

Guardian Cryptic #29608 | 15 Across | 2025-02-03 flower