Feeble king to use his brain (5)
I believe the answer is:
think
'use his brain' is the definition.
(thinking is using one's brain)
'feeble king' is the wordplay.
'feeble' becomes 'thin'.
'king' becomes 'K' (abbreviation for king in chess).
'thin'+'k'='THINK'
'to' is the link.
(Other definitions for think that I've seen before include "Judge, consider" , "Cogitate; deem" , "Use one's mind" , "Imagine" , "Meditate" .)