Traitor on fraternity (3)

I believe the answer is:

rat

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

'traitor' is the definition.
(I've seen this before)

'fraternity' is the wordplay.
'RAT' is hidden within the letters.
But, I'm uncertain how this is indicated.

'on' acts as a link.

Can you help me to learn more?

(Other definitions for rat that I've seen before include "Domestic rodent" , "one not going down with ship" , "Rodent; traitor" , "Plague-carrying rodent" , "one who deserts the sinking ship?" .)

I've seen this clue in The Mirror.

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