Slender king's figure (5)
I believe the answer is:
think
'figure' is the definition.
(I've seen this before)
'slender king's' is the wordplay.
'slender' becomes 'thin' (synonyms).
'king' becomes 'K' (abbreviation for king in chess).
'thin'+'k'='THINK'
(Other definitions for think that I've seen before include "Use one's mind" , "Cogitate; deem" , "Ponder" , "Meditate" , "Imagine" .)