River Ure guide undergoes training, as is proper (2,7)
I believe the answer is:
de rigueur
'is proper' is the definition.
(I've seen this before)
'river ure guide undergoes training' is the wordplay.
'river' becomes 'r'.
'undergoes training' is an anagram indicator (letters trained or shaped into a new form).
'r'+'ure'+'guide'='rureguide'
'rureguide' with letters rearranged gives 'DE RIGUEUR'.
'as' is the link.
(Other definitions for de rigueur that I've seen before include "Proper" , "Obligatory" , "Strictly" , "necessary" , "Required by strict etiquette or fashion (French)" .)