That royal quality about a girl (5)
I believe the answer is:
regal
'that royal' is the definition.
(I've seen this in another clue)
'about a girl' is the wordplay.
'about' becomes 're' (regarding).
'a girl' becomes 'gal' ('gal' can be a synonym of 'girl').
're'+'gal'='REGAL'
'quality' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for regal that I've seen before include "Portable organ" , "Showing great dignity" , "Kingly glare?" , "Royal - lager (anag)" , "King's" .)