Discover king in play with knight (5)
I believe the answer is:
learn
'discover' is the definition.
(I know that discover can be written as learn)
'king in play with knight' is the wordplay.
'king in play' becomes 'Lear' (Shakespeare's King Lear).
'with' means one lot of letters go next to another.
'knight' becomes 'n' (chess abbreviation).
'lear'+'n'='LEARN'
(Other definitions for learn that I've seen before include "Ascertain" , "Gain knowledge or skills" , "Comprehend" , "Succeed in school" , "Be educated" .)