Stuart queen is turning brown (6)
I believe the answer is:
sienna
'brown' is the definition.
(I've seen this before)
'stuart queen is turning' is the wordplay.
'stuart queen' becomes 'anne' (I've seen this before).
'turning' is a reversal indicator.
'anne'+'is'='anneis'
'anneis' reversed gives 'SIENNA'.
(Other definitions for sienna that I've seen before include "yellowy-brown" , "Earthy colour" , "reddish brown" , "Earth colour containing ferric oxides used as a pigment" , "Italian city and brownish pigment" .)