Pick up knight behind king (5)
I believe the answer is:
learn
'pick up' is the definition.
('learn' can be a synonym of 'pick up')
'knight behind king' is the wordplay.
'knight' becomes 'n' (chess).
'behind' is a charade indicator (letters next to each other).
'king' becomes 'lear' (I've seen this before).
'n' after 'lear' is 'LEARN'.
(Other definitions for learn that I've seen before include "Commit to memory" , "Master" , "Get to know" , "Discover - study" , "Study or digest" .)