A downhole tool, configured to receive a milling tool or other forcing tool, includes: a tubular mandrel; an adapter housing coupled to the mandrel; a guide sleeve disposed within the adapter housing; a movable sleeve configured for sliding movement within the guide sleeve; and a retainer positioned uphole of the movable sleeve. The retainer includes an upper annular portion, a lower annular portion, an annular void between the upper and lower annular portions, and a bridge portion extending between the upper and lower annular portions. The upper annular portion is initially fixed to the guide sleeve. The lower annular portion is configured such that downward movement of the lower annular portion causes the movable sleeve to move downward within the guide sleeve. The bridge portion comprises a through-passage and a thin walled segment adjacent to the void. Milling or otherwise disconnecting the bridge portion permits the lower annular portion to move the moveable sleeve.