Provider of beef under an age BSE affected (8,5)
I believe the answer is:
aberdeen angus
'provider of beef' is the definition.
(Aberdeen Angus is a type of cow)
'under an age bse affected' is the wordplay.
'affected' indicates an anagram.
'under'+'an'+'age'+'bse'='underanagebse'
'underanagebse' with letters rearranged gives 'ABERDEEN ANGUS'.
(Other definitions for aberdeen angus that I've seen before include "Black, hornless breed of cattle" , "Scottish black bovine" , "Joint supplier" , "Breed of beef cattle" , "Neat" .)