Female teacher hides master's mobile (12)
I believe the answer is:
headmistress
'female teacher' is the definition.
(a female headteacher)
'hides master's mobile' is the wordplay.
'mobile' indicates anagramming the letters.
'hides'+'masters'='hidesmasters'
'hidesmasters' anagrammed gives 'HEADMISTRESS'.
(Other definitions for headmistress that I've seen before include "Person in charge of a school" , "School principal" , "Female school leader" , "Teacher" .)