What does the cryptic crossword clue "Old, imprisoned? One escapes, still able to achieve goal (6)" mean?

The definition is "goal". The wordplay uses a deletion. O (synonym="old") + NSIDE (synonym="imprisoned") = ONSIDE The answer is ONSIDE.

Old, imprisoned? One escapes, still able to achieve goal (6)

Guardian Cryptic #29168 | 20 Down | 2023-09-06 flower