Condition favouring face covering, mostly (7)
I believe the answer is:
'condition' is the definition.
(I know that proviso is a type of condition)
'favouring face covering mostly' is the wordplay.
'favouring' becomes 'pro' (I've seen this before).
'face covering' becomes 'visor'.
'mostly' means to remove the last letter.
'visor' with its final letter removed is 'viso'.
(Other definitions for proviso that I've seen before include "Condition of stipulation of an agreement" , "Condition for VIP or so" , "Something insisted upon as part of an agreement" , "Requirement" , "Stipulation (in contract)" .)