Well, don't put it on a bonfire !
Be VERY careful indeed if you've 'adopted' this item, 5-0 Duce !! The provenance is unknown, it could be an illegal vet bring-back which has been quietly dumped.
Deactivation of a mortar round usually involves removing the fuze and emptying out the filling. If only half the fuze remains, it doesn't sound good.
Two options :
a) Take it to the police ( and risk losing the item altogether )
b) Seek out a friendly registered firearms dealer, or militaria dealer who knows their stuff, and ask their advice.
But -
be very careful with that thing !!
( Note : Where shells, bombs and ordnance are concerned, I'd really recommend dealing only with reputable dealers who sell certificated, deactivated items ).