Consider ousting king that's weak (4)
I believe the answer is:
thin
'weak' is the definition.
(similar in meaning)
'consider ousting king' is the wordplay.
'consider' becomes 'think' (I've seen this before).
'ousting' suggests deleting specific letters.
'king' becomes 'K' (chess abbreviation).
'think' with 'k' taken out is 'THIN'.
'that's' acts as a link.
(Other definitions for thin that I've seen before include "Skinny" , "Meagre" , "Unconvincing" , "Measure" , "Slim - watery" .)