Weak royal judgement (8)
I believe the answer is:
thinking
'judgement' is the definition.
(thesaurus)
'weak royal' is the wordplay.
'weak' becomes 'thin' (similar in meaning).
'royal' becomes 'king' (I've seen this before).
'thin'+'king'='THINKING'
(Other definitions for thinking that I've seen before include "Thought" , "rationale" , "Considering in the mind" , "consideration" , "Rational" .)