Comparatively wet ruler of principality (7)
I believe the answer is:
rainier
I believe this clue is a double definition.
'comparatively wet' is the first definition.
('rainy' can be similar in meaning to 'wet')
'ruler of principality' is the second definition.
(as in Rainier III, Prince of Monaco)
(Other definitions for rainier that I've seen before include "weather getting worse" , "Prince . . . . . . . rules Monaco" , "Former prince of Monaco" , "More wet - Monaco's late prince" , "more cats and dogs?" .)