Queen isn’t commonly old-fashioned (6)
I believe the answer is:
quaint
'old-fashioned' is the definition.
(cutely old-fashioned)
'queen isn't commonly' is the wordplay.
'queen' becomes 'qu' (abbreviation for queen).
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'qu'+'aint'='QUAINT'
(Other definitions for quaint that I've seen before include "'Olde-worlde'" , "Curious" , "Twee" , "Funny and old-fashioned" , "Attractively odd" .)