4: Experience a lack of oxygen, perhaps
5: Balance sheet positive
11: Emulate a couch potato
15: Professionals who might get under your skin?
20: Cry over spilled milk
25: Loses weight (with 'down')
26: Ship's positioning system
28: Himalayan monasteries
35: Gigantic 'Arabian Nights' flyer
51: Chamber music instrument
53: Bridle path excursion
56: Popular ABC adventure series