...seeing retainer beyond a river (9)
I believe the answer is:
observant
'seeing' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'retainer beyond a river' is the wordplay.
I cannot really understand how this works, but
'retainer' could be 'servant' (synonyms) and 'servant' is found within the answer.
'river' could be 'ob' (Ob is an example) and 'ob' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for observant that I've seen before include "taking things in" , "with a keen eye" , "Likely to notice" , "Good at noticing" , "Sharp-eyed" .)