Detention of gunman finally in act of disposing of body (10)
I believe the answer is:
internment
'detention' is the definition.
(internment is a kind of detention)
'gunman finally in act of disposing of body' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for internment that I've seen before include "Locking up" , "Detention" , "Committal" , "Confinement of an enemy national" , "Precautionary measure" .)