I'm impressed — infantryman becomes royal favourite! (5)

I believe the answer is:

corgi

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

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

I don't understand how the remainder of the clue works.

(Other definitions for corgi that I've seen before include "Pre-2018 royal dog" , "Small Welsh dog with fox-like head" , "Welsh 11" , "Welsh breen of dog" , "found in the kennels" .)

I've seen this clue in The Independent.

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.