Vain rules, unexpectedly widespread (9)
I believe the answer is:
universal
'widespread' is the definition.
(I've seen this before)
'vain rules unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'vain'+'rules'='vainrules'
'vainrules' is an anagram of 'UNIVERSAL'.
(Other definitions for universal that I've seen before include "Widespread, worldwide" , "Applying throughout the world" , "All-inclusive" , "Of worldwide scope" , "Everywhere applicable" .)