Crossword Genius

Keen-eyed ancillary on Russian river (9)

Ross

I believe the answer is:

observant

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

'keen-eyed' is the definition.
(thesaurus)

'ancillary on russian river' is the wordplay.
I cannot quite understand how this works, but
'river' could be 'ob' (Ob is an example) and 'ob' is found within the answer.
The remaining letters 'servant' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for observant that I've seen before include "Alert" , "Perceptive" , "diligent in duty" , "adhering to religious practices" , "Quick to notice things" .)

I've seen this clue in The Independent.
Want a hint initially instead of a full solution? Install my app