Guiding principle of Midas's reign? (6,4)
I believe the answer is:
golden rule
'guiding principle' is the definition.
(I've seen this before)
'midas's reign?' is the wordplay.
'midas's' becomes 'golden' (I can't explain this - if you can you should believe this answer much more).
'reign?' becomes 'rule' ('rule' can be a synonym of 'reign').
'golden'+'rule'='GOLDEN RULE'
'of' acts as a link.
(Other definitions for golden rule that I've seen before include "Basic principle to be followed in all cases" , "Midas had" , "Vital principle" , "Important principle" , "Guiding principle" .)