Hail regal leader of state (4)
I believe the answer is:
aver
'state' is the definition.
(thesaurus)
'hail regal leader' is the wordplay.
'hail' becomes 'ave' (I've seen this before).
'leader' indicates taking the first letters.
The initial letter of 'regal' is 'r'.
'ave'+'r'='AVER'
'of' is the link.
(Other definitions for aver that I've seen before include "make a statement" , "State firmly, declare" , "State decisively" , "State firmly, perhaps rave" , "Assert, state as true" .)