Underweight ruler’s philosophy? (8)
I believe the answer is:
thinking
'philosophy?' is the definition.
(thesaurus)
'underweight ruler's' is the wordplay.
'underweight' becomes 'thin' (similar in meaning).
'ruler' becomes 'king' (king is a kind of ruler).
'thin'+'king'='THINKING'
(Other definitions for thinking that I've seen before include "consideration" , "Take" , "rationale" , "giving opinion" , "Reasoning in the mind" .)