An animal with horns turns up on foot (5)
I believe the answer is:
ankle
'foot' is the definition.
(thesaurus)
'an animal with horns turns up on' is the wordplay.
'animal with horns' becomes 'elk' (I've seen this in another clue).
'turns up' is a reversal indicator.
'on' means one lot of letters go next to another.
'elk' written backwards gives 'kle'.
'an'+'kle'='ANKLE'
(Other definitions for ankle that I've seen before include "Joint between foot and leg" , "Foot-leg connector" , "Lower leg joint" , "Major joint in lower limb" , "Leg-joint" .)