Stalwart Republican in reading room (6)
I believe the answer is:
sturdy
'stalwart' is the definition.
(I know that stalwart can be written as sturdy)
'republican in reading room' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in' means one lot of letters goes inside another.
'reading room' becomes 'study' (room for reading or working in).
'r' placed within 'study' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Solidly made" , "Strong, solid" , "Uncompromising" , "Robust, strong" , "Strong and sturdily built" .)