Suspect mailman is into carnality (9)
I believe the answer is:
animalism
'carnality' is the definition.
(thesaurus)
'suspect mailman is into' is the wordplay.
'suspect' is an anagram indicator.
'into' is an insertion indicator.
'mailman' is an anagram of 'animalm'.
'animalm' going around 'is' is 'ANIMALISM'.
(Other definitions for animalism that I've seen before include "Maim snail (anag)" , "Imam slain (anagr.)" .)