Ham bone, of sorts, slightly decomposed (5)
I believe the answer is:
thigh
'ham' is the definition.
(I've seen this before)
'bone of sorts slightly decomposed' is the wordplay.
'bone of sorts' becomes 'T' (as in a T-bone steak).
'slightly decomposed' becomes 'high' (high can mean beginning to smell or decompose).
't'+'high'='THIGH'
(Other definitions for thigh that I've seen before include "Part of member" , "part of body" , "This bone is the femur" , "Ham" , "top of pin" .)