This appetite is unusual in wild deer (6)
I believe the answer is:
desire
'this appetite' is the definition.
(I've seen this in another clue)
'is unusual in wild deer' is the wordplay.
'unusual' is an anagram indicator.
'in' is an insertion indicator.
'wild' is an anagram indicator.
'is' with letters rearranged gives 'si'.
'deer' anagrammed gives 'dere'.
'si' placed into 'dere' is 'DESIRE'.
(Other definitions for desire that I've seen before include "Strong wish" , "Want to reside?" , "Want, wish for" , "Reside (anag.)" , "Lust" .)