Some defiant infants are quite hostile (4)
I believe the answer is:
anti
'hostile' is the definition.
(thesaurus)
'some defiant infants are quite' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ANTI' is hidden in the letters of 'defiant infants quite'.
(Other definitions for anti that I've seen before include "Someone opposed (to a policy?)" , "Who pooh-poohs all" , "Against, informally" , "Naysayer" , "One who is against (something)" .)