Bloke with pistol has terrorised treatment centre (4,8)

I believe the answer is:

guys hospital

I'm a little stuck... Click here to teach me more about this clue!

'bloke with pistol' is the definition.
I can't judge whether this defines the answer.

'terrorised treatment centre' is the wordplay.
'terrorised' becomes 'guys' (I can't justify this - if you can you should believe this answer much more).
'treatment centre' becomes 'hospital' (I've seen this before).
'guys'+'hospital'='GUYS HOSPITAL'

'has' is the link.

Can you help me to learn more?

(Another definition for guys hospital that I've seen is " London medical teaching establishment".)

I've seen this clue in The Times.

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