Undecided about British animal vet treated ... (10)
I believe the answer is:
ambivalent
'undecided' is the definition.
(thesaurus)
'british animal vet treated' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'treated' indicates anagramming the letters.
'b'+'animal'+'vet'='banimalvet'
'banimalvet' is an anagram of 'AMBIVALENT'.
'about' acts as a link.
(Other definitions for ambivalent that I've seen before include "With conflicting emotions" , "Showing conflicted attitude" , "Having mixed feelings (towards something)" , "with conflict in mind" , "Having opposing attitudes" .)