Take apart, since concealed by mask (11)
I believe the answer is:
disassemble
'take apart' is the definition.
('disassemble' can be a synonym of 'take apart')
'since concealed by mask' is the wordplay.
'since' becomes 'as' (I've seen this in other clues).
'concealed by' indicates putting letters inside.
'mask' becomes 'dissemble' ('dissemble' can be a synonym of 'mask').
'as' inserted within 'dissemble' is 'DISASSEMBLE'.
(Other definitions for disassemble that I've seen before include "Take to pieces" , "Take apart" , "Pull to pieces" .)