You can't go when you see one, perhaps (9)
I believe the answer is:
urologist
'you' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?
'can't go when you see one perhaps' is the wordplay.
I cannot quite understand how this works, but
'see' could be 'lo' (archaic exclamation meaning 'look!') and 'lo' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for urologist that I've seen is " Doctor specialising in the waterworks?".)