Spaniard with a large upper-class Welsh girl (10)
I believe the answer is:
andalusian
'spaniard' is the definition.
(Andalusia is a region of Spain)
'with a large upper-class welsh girl' is the wordplay.
'with' becomes 'and'.
'large' becomes 'L' (eg in clothes sizes).
'upper-class' becomes 'u' (similar in meaning).
'welsh girl' becomes 'sian' (common Welsh name).
'and'+'a'+'l'+'u'+'sian'='ANDALUSIAN'
(Other definitions for andalusian that I've seen before include "Spanish" , "Chicken" .)