I name previous rat (8)

I believe the answer is:

informer

'rat' is the definition.
('informer' can be a synonym of 'rat')

'i name previous' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'previous' becomes 'former' ('former' can be a synonym of 'previous').
'i'+'n'+'former'='INFORMER'

(Other definitions for informer that I've seen before include "who may betray someone?" , "One aiding police" , "Criminal helping police" , "He gives incriminating data about others to the police" , "sneak" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.