Orb emptied by retainer on the guard (9)

I believe the answer is:

observant

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

'on the guard' is the definition.
I can't tell whether this defines the answer.

'orb emptied by retainer' is the wordplay.
'emptied' suggests removing the centre.
'by' means one lot of letters go next to another.
'retainer' becomes 'servant' (synonyms).
'orb' with its centre removed is 'ob'.
'ob'+'servant'='OBSERVANT'

Can you help me to learn more?

(Other definitions for observant that I've seen before include "with a keen eye" , "Good at noticing" , "Alert" , "aware of surroundings" , "(Of music)using conventional keys" .)

I've seen this clue in the Sydney Morning Herald.

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.