Consider slender king (5)
I believe the answer is:
think
'consider' is the definition.
(I know that consider can be written as think)
'slender king' is the wordplay.
'slender' becomes 'thin' (similar in meaning).
'king' becomes 'K' (chess abbreviation).
'thin'+'k'='THINK'
(Other definitions for think that I've seen before include "Believe" , "figure" , "Imagine" , "Cogitate; deem" , "Use one's mind" .)