What does the cryptic crossword clue "You can't go when you see one, perhaps (9)" mean?

The definition is "You can't go when you see one, perhaps". The wordplay uses a cryptic definition. Cryptic definition The answer is UROLOGIST.

You can't go when you see one, perhaps (9)

Guardian Cryptic #29138 | 25 Across | 2023-08-02 flower