Regularly test a Republican leader (4)
I believe the answer is:
tsar
'leader' is the definition.
(I've seen this before)
'regularly test a republican' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'republican' becomes 'r' (abbreviation).
The alternating letters of 'test' are 'ts'.
'ts'+'a'+'r'='TSAR'
(Other definitions for tsar that I've seen before include "Absolute ruler" , "Autocrat" , "Eg, Ivan the Terrible" , "Title of Russian emperor" , "Russian ruler (S, not Z)" .)