Staunch Republican in office (6)
I believe the answer is:
sturdy
'staunch' is the definition.
(thesaurus)
'republican in office' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in' indicates putting letters inside.
'office' becomes 'study' (I've seen this before).
'r' going within 'study' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Strong and sturdily built" , "Strong and reliable" , "Stalwart, stout, tough" , "Robust, strong" , "Solidly made" .)