Day Welsh girl adopts English milk supplier (8)
I believe the answer is:
friesian
'milk supplier' is the definition.
(Holstein Friesian is a breed of cow)
'day welsh girl adopts english' is the wordplay.
'day' becomes 'fri'.
'welsh girl' becomes 'sian' (common Welsh name).
'adopts' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'fri'+'sian'='frisian'
'frisian' enclosing 'e' is 'FRIESIAN'.
(Other definitions for friesian that I've seen before include "Source of beef" , "from Holland" , "Monochrome cow" , "Milk producer" , "source of milk" .)