Pressure from headmistress? Not half! (6)
I believe the answer is:
stress
'pressure' is the definition.
(I've seen this in another clue)
'headmistress? not half' is the wordplay.
'not half' indicates taking half.
'headmistress' cut in half is 'STRESS'.
'from' acts as a link.
(Other definitions for stress that I've seen before include "possible excuse for sickie?" , "possible cause of fracture?" , "Emphasis: result often of being under pressure" , "why off work?" , "Highlight" .)