Animal may appear on cue (5)
I believe the answer is:
ounce
'animal' is the definition.
(I've seen this before)
'may appear on cue' is the wordplay.
'may appear' indicates anagramming the letters (how the letters might appear in a different order).
'on'+'cue'='oncue'
'oncue' with letters rearranged gives 'OUNCE'.
(Other definitions for ounce that I've seen before include "Unit equal to about 28 grams" , "scrap" , "Wild cat or light weight" , "Oz" , "one of several in a pound" .)