Rat that you'll find in canals in Geelong (4)
I believe the answer is:
sing
'rat' is the definition.
(both can mean to inform on someone)
'you'll find in canals in geelong' is the wordplay.
'you'll find in' says the answer is hidden in the clue.
'SING' is hidden in the letters of 'canals in geelong'.
'that' is the link.
(Other definitions for sing that I've seen before include "Croon" , "Vocalise musically" , "Celebrate" , "Turn informer" , "pipe" .)