Aim to sign evangelists (12)

I believe the answer is:

missionaries

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

'evangelists' is the definition.
('missionary' can be a synonym of 'evangelist')

'aim to sign' is the wordplay.
'aim' becomes 'mission' (I can't explain this - if you can you should believe this answer much more).
'to sign' becomes 'aries' (star sign).
'mission'+'aries'='MISSIONARIES'

Can you help me to learn more?

(Another definition for missionaries that I've seen is " ministers sent abroad".)

I've seen this clue in The Globe and Mail and The Guardian.

Want a hint initially instead of a full solution? Install my app