Spanish youth rejected by an upper-class Welsh girl (10)
I believe the answer is:
andalusian
'spanish' is the definition.
(Andalucia is a part of Spain)
'youth rejected by an upper-class welsh girl' is the wordplay.
'youth' becomes 'lad' (I've seen this before**).
'rejected' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'welsh girl' becomes 'sian' (common Welsh name).
'lad' back-to-front is 'dal'.
'dal' after 'an' is 'andal'.
'andal'+'u'+'sian'='ANDALUSIAN'
(Other definitions for andalusian that I've seen before include "Spanish" , "Chicken" .)