Discover knight sitting by river beyond meadow (5)
I believe the answer is:
learn
'discover' is the definition.
('learn' can be a synonym of 'discover')
'knight sitting by river beyond meadow' is the wordplay.
'knight' becomes 'n' (chess).
'sitting by' means one lot of letters go next to another.
'river' becomes 'r'.
'beyond' is a charade indicator (letters next to each other).
'meadow' becomes 'lea' (I've seen this before).
'r' after 'lea' is 'lear'.
'n' after 'lear' is 'LEARN'.
(Other definitions for learn that I've seen before include "Comprehend" , "Hear - memorise" , "Acquire information" , "Become aware of" , "Gain knowledge or skills" .)