Republican in Senate unusually sincere (7)
I believe the answer is:
earnest
'sincere' is the definition.
(synonyms)
'republican in senate unusually' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in' means one lot of letters goes inside another.
'unusually' indicates anagramming the letters.
'senate' anagrammed gives 'eanest'.
'r' inserted into 'eanest' is 'EARNEST'.
(Other definitions for earnest that I've seen before include "promise" , "Serious in intention" , "Something of value to bind a contract" , "Oscar Wilde wrote ''The importance of Being . . . . . . . '' (7)" , "Eastern (anag.)" .)