Coming across aliens about the cathedral? (5,10)

'coming across aliens' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'about the cathedral?' is the wordplay.
I cannot really see how this works, but
'about' could be 'c' (abbreviation for circa) and 'c' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'cathedral?' could be 'see' (I've seen this in another clue) and 'see' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?