Wolf upset plain herder (11)
I believe the answer is:
philanderer
'wolf' is the definition.
(I've seen this before)
'upset plain herder' is the wordplay.
'upset' indicates an anagram.
'plain'+'herder'='plainherder'
'plainherder' with letters rearranged gives 'PHILANDERER'.
(Other definitions for philanderer that I've seen before include "Casual lover" , "wolf" , "Male flirt" , "Womaniser, libertine" .)