One hides face as guest loses it (5)
I believe the answer is:
visor
'one hides face' is the definition.
(a visor hides a person's face)
'guest loses it' is the wordplay.
'guest' becomes 'visitor' (I've seen this before).
'loses' is a deletion indicator.
'visitor' with 'it' removed is 'VISOR'.
'as' acts as a link.
(Other definitions for visor that I've seen before include "eye-shield" , "Front piece of a helmet" , "Face-armour" , "Part of an armour helmet" , "Face-protector" .)