Ring head of state up, showing good judgment (7)
I believe the answer is:
resound
'ring' is the definition.
('resound' can be a synonym of 'ring')
'head of state up showing good judgment' is the wordplay.
'head of state' becomes 'er' (abbreviation for Elizabeth Regina).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'showing good judgment' becomes 'sound'.
'er' in reverse letter order is 're'.
're'+'sound'='RESOUND'
(Other definitions for resound that I've seen before include "Reverberate" , "Echo" , "ring" .)