Cover for face when headmaster breaks jaw (7)
I believe the answer is:
yashmak
'cover for face' is the definition.
(I've seen this before)
'when headmaster breaks jaw' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'headmaster' becomes 'hm' (I am not sure about this - if you are sure you should believe this answer much more).
'breaks' indicates putting letters inside (inserted letters break into the word).
'jaw' becomes 'yak' (both can mean to talk excessively).
'as'+'hm'='ashm'
'ashm' placed into 'yak' is 'YASHMAK'.
Can you help me to learn more?
(Other definitions for yashmak that I've seen before include "Veil concealing face worn by Muslim women" , "Veil worn by Muslim women in some countries" , "Am shaky about face-veil for Muslim woman" , "Eastern veil" , "Muslim women's veil" .)