Hail republic's leader, socialist declared (7)
I believe the answer is:
averred
'declared' is the definition.
(averring is a kind of declaring)
'hail republic's leader socialist' is the wordplay.
'hail' becomes 'ave' (greeting in Latin).
'leader' says to take the initial letters.
'socialist' becomes 'red' (socialism is often associated with the colour red).
The first letter of 'republics' is 'r'.
'ave'+'r'+'red'='AVERRED'
(Other definitions for averred that I've seen before include "Stated, asserted" , "confidently stated" , "Claimed" , "Stated to be the case" , "Alleged, asserted" .)