Love to upset an animal (4)
I believe the answer is:
vole
'an animal' is the definition.
(thesaurus)
'love to upset' is the wordplay.
'to upset' indicates anagramming the letters.
'love' is an anagram of 'VOLE'.
(Other definitions for vole that I've seen before include "A little rodent" , "Strange love for a rodent" , "win all the tricks" , "Love this rodent, strangely" , "Holm-oak" .)